I have a gmp075-3 installed 2000 I have noticed this summer the blower will come on by itself even when turn off at thermostate didn't think nothing of it thought it was just hot up in the attic but now that it is cooled off when I went to turn the heat on the burner ingnite all three then 10 to 15 sec they shut down it will do this 3 times the on the circuit board it flashes 1 times slowly over and over I clean the flame sensor still doeing the same thing.I also ruled out the thermostate any help would greatly be appreciated. Thank you for your time

Usually the flame sensor causes this problem OR a grounding problem with the sensor or some other component. Take your meter and get a reading on the flame sensor so we know what it is putting out.