Currently trying to trace a problem where At 55+mph if you signal left the truck wants to shut down. I have tried a new flasher, the three prong flasher would only cycle constantly with no indicator or outside lighting. I had to put a 2 prong flasher in just to get some sort of signals and checked bulbs. i have replaced the entire turn signal switch and have no blown fuses. Any ideas/suggestions/schematics would be great as my chilton doesn't show everything.

(94 chevy k 2500 6.5l diesel 4x4)

Tim