Furnace keeps cycling on and off until it's up to set Temp
this could be caused by a few things. Does the main gas valve loose power for that time that it cycles off.  If it continues to receiver 24-volts during the entire cycle, the problem is in the main control valve. I has to be renewed.  If the power is interrupted, some thing is breaking the signal.  The transformer could be overheating and opening a coil momentarily. Jumper the red to the white on the board then find out where you are loosing the signal by jumpering across every component with the meter on a low ohms scale.  I have seen boards that will open intermittently.  A tarnished flame sensor will do this.  buff it good with scratchy pad of fine sand paper.   Good Luck.