I do not have a mixing valve on my boiler, will this create mixing problems? Sometimes it mixes ok. other times it is all hot or cold. I replaced the cartridge once. This system is only a couple of months old.

Does your boiler produce steam for living space heating purposes? Or is it actually just a water heater? Unless you are using the hot water heater/boiler to generate 180 degree hot water for a restaurant kitchen or the sort, your heater should be set to around 120 degrees at the highest. In which case you might want to install a tempering valve on the supply piping that furnishes your bathrooms with hot water. That's just a "might" want to, not a "should do so". My suggestion is to set the water heater around 100 degrees and call it good. Any properly functioning anti scald tub or shower valve is designed to work very well under this condition of supplied hot water.
You may be experiencing thermal expansion in your hot water piping and storage tank during times when the system is idle, not flowing or sending any hot water anywhere in the house/structure. In the event of expansion, hot water can replace every drop of cold water in the cold water supply piping, causing hot water to be dispensed from the cold water piping for a brief few seconds, maybe up to 10-15 seconds of hot water could be experienced in a drastically expanded system before the supply of fresh cold water has replaced the higher pressured hot water that has invaded the system.
This kind of expansion is caused by overly heated hot water and could be as simple as turning the temperature setting down on your water heater, or it could be a sign of a malfunctioning thermostat causing over heating of the water, especially during idle times at night. If this is the case, you had better check to make sure your pressure relief valve is working properly, otherwise a very unsafe situation will be knocking at your door.
It is very important that you check to make sure your water heater/boiler is cycling off. Turn the temperature setting to it's lowest setting and wait a few moments to see if it turns it's self off. The flames in the heat exchange chamber should go off within a few minutes if no one has been drawing any hot water in the house. If it never seems to go off....turn off the gas supply valve and have a Professional Plumber take a look at the unit.
Now...if all my worries are for not....here's how to prevent the hot water from expanding into the cold side when the system is idle during the night.....Install a check valve on the cold water supply piping that feeds the water heater/boiler......and install an expansion tank on the outlet side piping, (hot water side), of the heater. Turn your temperature setting down to 100-110 degrees F. on the water heater/boiler control valve. It might necessitate using a thermometer held under running hot water from a faucet to achieve a true desired setting. That will solve your expansion problem, as long as you don't have any other failing equipment or mixing valves that are malfunctioning, and failing to prevent a cross connection between the cold and hot water supplies.
Sometimes folks make the mistake of installing a shut off valve on the spout of their laundry sink faucet, leaving the hot and cold faucet valves open and preventing flow from the faucet by turning off the valve on the spout.
This is a perfect example of a cross connection....the hot water can over take the cold water in the supply piping to the sink faucet with it's much more powerful expanded and constantly increasing pressure.
A cross connection like this could easily cause the entire cold water supply piping to be replaced with hot water by the next morning. So make sure you don't have any equipment failures or self created cross connections that are adding to the cause of this chaos.
But most importantly, have a look at your water heater/boiler to make sure it is functioning properly.
Good luck and I can't express how important it is to check your water heating equipment......ASAP.

If your posi-temp valve is working correctly this should not be a problem. Does your hot water temp. ever get extremly hot or just warmer? The way the posi-temp cartridge is designed if you have a large drop jn cold water pressure then the inside piston is supposed to slide to the hot side restricting the flow of hot water to a safe level.If the pressure on the hot side drops suddenly then the valve is supposed to restrict cold water flow.
If the temperature gets to a unsafe level you can adjust maximum hot setting by removing the handle. There is a allen head set screw on the bottom of the handle. Loosen screw and pull handle off. On the newer models pull off chrome sleeve from around valve body and slide outer geared disc off enough to seperate from inner gear. There are markings on disc with arrow pointing to h. or c. Adjust outer disc in the direction you want to adjust and slide it back so that gears engage. Each tooth on gears will change water temp. about2-3 degrees.Try it and readjust till you get the hot set point you want. One other thing you could do is install a small pressure tank on cold water line to compensate for pressure changes. Hope yhis helps you. Thanks