I hope your problem is fixed by now. First the fan motor needs to be the same RPM as the old one. It can be a larger Horse power, but it the speed is too high it will cool the exchanger down too fast. Especially if the house is already cold. Now if it is a multispeed motor, generally you will use the lowest fan speed for heat and highest for air conditioning. The lowest speed is generally a red wire. The highest is usually black or blue. If you have this backwards you will have problems in the summer time. (That is why I decided to go ahead and respond in the off chance you will come back to this post.) From the days I used to work on gas furnaces (which has been about 7 years ago), the only two things that come to mind about a motor coming on and off is: The exchanger is getting too cool--which is due to a cold inside temp air, fan speed too high, or fan temp low limit set too high. Or two, motor going out on overtemp. This can be caused by not having enough air resistance--this is unusual, unless you went alot higher in horsepower. (Which would usually mean the motor won't even fit in the blower assembly. If you still have problems PM me and maybe I can help you some more. Good luck..