With all gauges looking ok, I too had the check gauge light popping on and off. After 2 months off guessing, it turned out to be a bad oil pressure sending unit. The seal in the sender was going bad and giving false readings. The gauge itself would sit about a 1/5 of the way up (close to the'n') One day the seal just gave out and started gushing oil over the front of the motor. Cheap easy fix and the gauge looked ike it read better too. Halfway up insted of lower. (I know its a dummy) Recently had the same thing happen again in my 90. I changed the oil because it needed it and the problem went away temporaraly (sp?). About 1000 miles later it came back. Change sensor...no problems in last 4k. Hope this helps. Mine would also throw the light mostly at highway speeds. Good luck.
Joe