Why would a furnace seem to cycle every 25-30 seconds?

check the flame sensor,it sounds like it has oxidation on it,remove it and use a cardboard fingernail file and remove the oxidation from it,put it back together and try it.the flame sensor is a 2 to 3 inch piece of metal with ceramic coating at the base.it will have a single wire coming off the bottom.should have a 5/16 hex head screw holding it in place.