Water pressure good, thermostat calling for heat boiler ignites , heats to 80, turns off, drops to 40 reignites - keeps this cycle going. temp sensor? return sensor? any help appreciated

Check flame sensor is clean . check pressure switch is working and connections are tight and hoses are not split check condensation pipe is not blocked

sounds like the hot water sensor is shutting down early .also check flame sensor is clean .

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

Pressure inside the tank will rise along with the water temperature. As the water inside cools the pressure will wane until the heating element turns on and reheats the water with a subsequent rise in pressure. Running the hot water at the same time will draw heat energy from the boiler and lower the internal temp and pressure as well.

It sounds like your thermostat could be the problem. Have you noticed the boiler cutting off at all, or does it run continuously until it goes out on low water?

You stated, "when boiler is on", so I am wondering how you get the boiler on. If your boiler is controlled by a thermostat, you need to check it for a cold draft possibly from behind the thermostat in the wall where the wires come through. The thermostat should never be on an outside wall. The thermostat could be defective.

An aquastat can also control your boiler. If you have an aquastat, check the setting, typically 175 - 180 degrees. You should also have a "high limit" which should be set at 210 - 220. Some people run that high side limit higher; not a good idea - defeats the purpose. If the aquastat is set too high, the boiler will operate too hot and the problem you have will occur.

"pressure rises and pressure relief valve opens" This could mean that your pressure switch is prematurely venting. Check boiler temperature to see exactly at what temperature the T&P valve purges. If it blows off below the high side setting, either adjust or replace. The T&P valves are usually not adjustable. That blow off temperature is stamped on the head of the valve. So, if it discharges too soon, replace it.

Have you actually seen evidence thgat the T&P valve is opening? You may have a bad bladder in your expansion tank and your system may have air in it. Your boiler should have 12 psi when cold and 20 - 22 psi when operating. Check your boiler pressure guage.

"this eventually drops below the minimum operating pressure" Technically, there is set point for a NO MINIMUM OPERATING PRESSURE. That is a function of the amount of water in the boiler. If the T&P is blowing off, it will vent water from the boiler and the boiler pressure will go down. There is a minimum operating TEMPERATURE.

Your boiler could be low on water. You will have to manually fill the boiler. Be careful of thermal shock to the boiler. Feel the piping, and fill only when the circulator is running. Make sure the water is not going directly into the boiler. The supply water should go into the circulating supply side which feeds the radiators, radiant floor, or base board circuits.

All this will adversely affect your hot water. The heat exchanger for the potable hot water may have scale build up in it. Using the hot water keeps the boiler running, and a scaled heat exchanger will give you cooler hot water and the boiler keeps running. If this is a problem, you will need to replace the heat exchanger. A better idea is to not use the boiler for your domestic hot water. Why run the boiler in the summer?

When water is heated it expands. To deal with this all boilers are fitted with a small expansion vessel which is like a reservoir which holds the excess water.

This expansion vessel is like a balloon in a metal pot. When the water goes in the metal pot the baloon squashes a bit to make room for the extra volume of water. When it cools the balloon pushes the water back into the heating system.

What has happened is your expansion vessel no longer works. When the water heats up there is nowhere for it to go so the pressure jumps up to 4 bar. The Worcester boiler lets high pressure water escape (most boiler allow this to go through the safety valve at say 3.5 bar).

When the water cools the pressure drops very low as the volume of water contracts.

The problem is your expansion vessel - normally a little red painted metal cylinder inside the boiler.

Your boiler has an expansion vessel that is like a bladder full of air. Yours has lost it's air pressure. Check your manual for this vessel and where the valve is to re-inflate it and what pressure to inflate to. It should have a simple tyre valve, pump it back up (usually to about 1 bar after releasing the water pressure) and should be all fixed. If you can't re-inflate the bladder is holed and the expansion vessel will need replacing.

thermostat on the gas valve needs to be replaced,, which you will probly have to change the hole gas vale ,, check on the parts and labor and if it is over 6 years old wheigh the diffrence between it and a new unit good luck

http://www.worcesterbosch.com/Go there see if you can get some help.

i think you will find that instead of modulating as demand requires the boiler is overheating and then re operating after the safety thermostat has operated.
i wilud put some de scaler into the main heat exchanger circuit or add it to the radiators and let it circulate for a while.
also make sure the pump is operating properly to ensure proper water flow.