electrical problems on a 1993 1500 ck pickup

I have a 1993 Chevy 1500 pickup and i'm having some problems with the electrical system. It runs fine, engine wise, but when i go to turn on the ac or anything the uses power all the gauges go dead and the
speedometer starts bouncing. If i turn the radio on its goes with the beat if i turn on the turn signals it goes with it. I've checked my alternator, battery and starter and their all fine. Any help would be great guys thanks for your help

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