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sounds like a bad case of VD, voltage drop. probably circuitry is shat to hell. i got a cluster from a dx with no tach, PM me if you want it, cuz i sure dont.
Honestly, it probably is a voltage drop. I'd replace the engine grounds, or add one, and see if that takes care of the problem. It seems lame, but D engines are really touchy with that crap. I've done enough swaps that wouldn't start because of grounds to know...