The pressure in my boiler is too high and the relief valve is leaking and then opening.

Change the pressure relief valve, it staying open.

do you have a pressure tank as part of your water heater and water pipes? if yes, check to make sure tank pressure matches your incoming water pressure from your municipal hook up

I assume losing pressure through the PR means Pressure Relief Valve (PRV) or temperature pressure (TPR). Open in TPR may mean boiler is overheating/overpressurized. Open in PRV means the Pressure Reducer/Fill Valve in the fill line of the boiler is broken.
House Water Pressure in a home is 30/40 PSI min to 60-80+PSI max.
Boiler PRV/TPR are set at 30 PSI open (SYSTEM PRESSURE) so too high a house water pressure will open the PRV/TPR. Replace the reduction regulator on/in the fill valve assembly

Hot water pressure relief valves serve two functions. The first is to remove high-pressure water from the hot water tank. The second function is to act as a high temperature valve to remove water that exceeds a predetermined point for the valve. Both of these relief settings are preset at the valve factory. Many pressure relief valves will come with a metal tag that identifies these release points. If you're attempting to remove the hot water heater pressure relief valve, pay careful attention to ensure that you replace with the correct valve.

Your hot water heater has a pressure relief valve safety mechanism. When pressure inside the water tank exceeds certain limitations, the valve opens up to allow the pressure to be released. A faulty pressure relief valve will not release excess pressure, which could cause extensive damage to your home. You should test the hot water heater pressure relief valve once every six months to ensure proper operation. If the pressure relief valve fails the test, replace the valve with a new one immediately

Locate the pressure relief valve on your hot water heater. The pressure relief valve has a lever handle on top with a drain pipe attached to it. The pressure relief valve is most often on the side of the water tank toward the top. Newer models of hot water heaters have the pressure relief valve on top of the water tank.

Inspect the area around the pressure relief valve for signs of leaking water. If there is leaking water near the relief valve connection, replace the relief valve. If no leaking is evident, proceed with testing the pressure valve.

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Examine the area below the pressure relief valve drain pipe. Most water heater closets either have a drain in the floor below the pipe or the pipe is inserted into another pipe to direct the water out of the home. If the end of your drain pipe is not in another pipe or there is no drain in the floor, place a bucket under the drain pipe to catch water when you test the pressure relief valve.

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Pull the lever handle on the top of the pressure relief valve up with your fingers. A working lever will lift up without difficulty.You should begin to hear water going through the drain pipe. Slowly release the lever to shut off the pressure relief valve. The pressure relief valve is good. If you did not hear water running through the drain pipe, lower the lever on the pressure relief valve and replace the valve. If the lever is difficult to raise, do not force the lever. Replace the pressure relief valve.

Now with all that said if you have a closed loop water supply system and you have a pressure vessal/expantion tank it may have gone bad and is making the TP valve leak. you can call your water provider to see if you have closed loop system.

It's gas operation only, but has electronic ignition for propane. ( just to confirm, if you look at back of water heater, it would have a metal or black plastic cover and hydro cable going in side or top of it.)
There are usually 3 things that may cause pressure relief to leak. 1) Faulty,corroded pressure relief valve, which usually needs to be replaced if it goes faulty. 2) water temperature is getting too hot, which could indicate a faulty thermostat. 3) If your hooked up to city water, the pressure may be too high, causing excess pressure build up in hot water tank, thus pressure relief opens. If your running off your on board pump, this is probably not the cause, as they are usually set at 40 psi.
Ist, - try pulling lever on relief valve slightly open ( careful it's hot !!) to clean out any calcium that may be logged on seal thus causing leak. If that doesn't do it, and the hot water isn't running out steady and appear to be under extreme pressure while leaking, then I'd change the pressure relief valve. Remember when you do, shut off water to system, and relieve pressure by opening a tap, and pressure relief as well after that. New one shouldn't be too expensive, just make sure you get same thread, 1/2"NPT or 3/4"NPT. Use a little teflon tape, or pipe dope whatever you prefer when re-installing as well as clean threads a little before.
If it really starts gushing out pressure relief when heating or heated, and water seems hotter than normal, it's more than likely a faulty thermostat. I'll wait for your reply on what condition your experiencing before I go in to how to change the thermostat.

Pressure relief valve - replace
The internal seal gets bunged up with dirt & scale and then leaks water [pressure] to that outside pipe
To avoid draining the whole system, turn off the two central heating taps on the 3/4 pipes and then operate the valve manually and / or disconnect - only a small amount of water will escape / drip to below the boiler
Replace valve and fill as before.
If you think your system needs flushing out - now is a good time
Turn off mains electricity to boiler before starting work

If there is no leak anywhere in the system [pipes & radiators] Check the emergency pressure relief valve. It usually is at the base of the boiler and drains to outside [15mm pipe] where, if the valve leaks, water drips can be seen.. Looks similar to the picture with your query
Renew the valve
You can take it apart and clean it but I've never found that to be successful in the long term

There is a 15mm pipe from the pressure relief valve which should so through to the outside. These valves are prone to leak after time. Look at the end [outside] of this pipe. Wet = leak from valve

Whining = pump bearings
Pressure reduction = pressure relief valve diaphragm either perished of gunged up - look for water coming out of the overflow pipe from the valve
leak in ch system look at all joints

Under the boiler a little right of center there is a blue/other colour knob. Turn this anticlockwise [as you look up] [twist from right to left] This is the mains inlet tap to let water into the boiler and create the pressure shown on the guage 0.5 bar is the absolute nin.
allow the water to flow in until the guage reads about 1 bar then turn off the knob -not too tight as these are plastic
If you allow too much pressure- do not worry, there is an automatic relief valve.
If the boiler persists in loosing pressure, look for a leak in all pipes to & from radiators. If no leak then check the pressure relief valve outlet pipe, it usually goes through the wall to the outside. Look at the end for water drips & if there are drips replace valve