Gas and Oil Home Heating Furnaces - oil furnace Unicell

I've been having problems the last few years with the fan timer going bad.(Located in the upper part of the heater by the squirrel cage). I got tired of paying high costs for replacement and installed my own.There's a reset located in this same area.(This is a downdraft heater w/AC underneath)forced air. I also replaced the old thermostat with a Honeywell RTH6300B. I programmed the new thermostat according to directions. The heater works ok until at some point it kicks out and blows cold air. I need to press the reset button up by the squirrel cage again to make the burner come on. I'm thinking that this sensor I'm resetting needs replacing. The heater has been serviced two years running...
I'm used to older systems without electronic parts and have repaired these older systems many times. I have good electrical knowlege and have a meter, but no knowledge of the new systems and what to look for with this problem.
What do you think causes my heater to work just fine and then after it restarts,runs until it blows cold air again and I have to press the reset again?

Thanks,
Mike

That reset you are pushing is a high temperature safety. The most common cause for these tripping is lack of air flow.

Check the air filter, blower wheel, & A/C coil to be sure they are clean. If you recently installed one of those high efficiency pleated air filters, take it out & try the furnace.

Something else to check is make sure all registers are open & not restricted.