Hi, I had the same problem. Out of the blue Fuse 904 was blown, and left upper flipper did not work any more.
Replaced the fuse but after a few games it was blown again.
I tested the flipper with the left button a felt a shocking movement, tested it in the testmenu and I had full power.
Then I cleaned the left opto and the flipper is working like a new one.
Problem solved, and perhaps I can beat my old record now!