Gas and Oil Home Heating Furnaces - Janatrol Furnace won't stay lit

I have a Janatrol / Goodman GMP 150-5 that will not stay lit. It goes through all steps perfectly until approximately 30 seconds after start of burn. At that point, the flame goes out ant the indoor fan starts simultaneously. During the 2nd, 3rd and 4th attempts, the indoor fan is already running but the flame goes out 30 seconds into burn. I have checked / jumped all rollout and over limit switches. They are fine. I have cleaned the flame sensor. I have noticed that the light on control board goes out for a fraction of a second when the flame goes out but does not flash any codes, just the on-off-on one time and then back to continually on. This unit has AC on it and that is working fine. My question is, what would be the next thing to check? Would the transformer go low voltage at the time the relay tell the indoor fan to turn on? Does the unit tell the fan to turn on even if it is already running? Should I somehow test the flame sensor? (How?) Thanks in advance for any help.
PS I have watched this thing for about 200 cycles, I know it very well at this point.

check the ground.

Tinner73 - You nailed it. I was checking the grounds and found where someone had tied three grounds together. There was no lead going to chassis ground, only to the COM on the IC board. I think the whole thing was trying to ground through the transformer ground. I added a chassis ground and everything is now working or at least testable. After I established a good ground, I found a leaking PV pressure hose. I think this fixes a ongoing short cycling problem that I have had for years. Thanks for your help.