Gauge issues?

ok, I have a 75 duster, I have been having an issue with my temp and fuel gauge's. I did fry my old fuel gauge testing it. I had to replace the fuel sending unit when I cleaned out the fuel tank. I also bought some used gauges and replaced that voltage regulator when I swapped out gauges. For about two week after my temp worked ok, however my fuel gauge was never accurate. I did some testing to see if the wire had a bad ground, I did find that the voltage out of the main harness was wacky. Then one day I shut off the car, and the next day, everything was back the way it was before I fried the fuel gauge. I think the circuit board might be bad, however I am wondering if there is something I should check first. I f it the circuit board I am switching to all aftermarket mechanical gauges, if its something simple I wont. any help would be appreciated.

Thanks