Kind of an odd deal here, but long story short I recently installed a new-to-me 88' MY cluster in my 84' T/A, when I did this I was very meticulous with wiring and used good grounds etc. I installed an aftermarket GPS speedometer sending unit and calibrated it to the factory speedo and it works perfectly, but only 60% of the time. The other 40% it either reads 0MPH or, for lack of a better desc., tries to sweep but doesn't make it and bounces up and down from 0 almost like it isn't getting a strong enough input signal. The other issue is the tachometer reads erratic, sometimes it works perfect, and other times it bounces from the correct RPM back down to 0 back and forth, and it also sweeps/bounces in rhythm with the wipers and blinkers when activated. The car has an Autometer MonsterTach installed (which functions perfectly), and I ran the signal wire directly from that tach's input to the factory tach. I am wondering if I just have some goofy short or electrical gremlin somewhere or, if my tachometer signal needs to be modified. I need to double check but I believe the tach signal is using my MSD 6AL ignitions tach ouput currently, maybe I should try installing the MSD tach signal adapter? Is our OEM tach considered "voltage-triggered?" Any help is much appreciated, I am scratching my head on this one. I have double checked grounds, good 12v connections, no obvious shorts, etc.