Help can't figure out why I lose power when use my lights or turn signals

At an idle it's 13.96 and with accessories running it's 15.2. How to put two new Alternators on it tried a new ECM new coil new ignition switch new battery checked fuses put a new gauge cluster in it and a new starter and still have got no change

When you say " lose power" , you are referring to electrical power , NOT engine power , correct?--If that's the case , I'm picturing that you mean the lights get dim and the gauges fluctuate?? IF when the 'power loss' occurs , the alternator voltage is not changing , I would suspect maybe a ground issue. Any body work in the past?

It's never done this before and the lights just don't dim it kills the motor and the turn signals make them motor surge I have redone done all the grounds. It started this one day after I got gas just out of the blue if I accelerate up a steep hill or Mountain Pass everything correct itself until I start coming off the pass then it all starts happening again

Wish I could see this first-hand!! Sounds like the stuff l like to deal with, unfortunately that does you no good! My only thought right now , would be to try and isolate circuits as best as possible , it IS a '91? Will it keep running with the alternator unplugged?Then what?