94 International speedo/tach not working

So I took it to a different mechanic. They accidentally fixed it. They had messed up some other gauges while trying to fix the speedometer. While they were trying to fix the other gauges they fixed the speedometer. They put a different, newer instrument cluster on it and it worked. I don't know if they did something else, but that's what they told me. I just got it back this morning.

 

Seemed I spoke to soon about it being fixed. I got in the truck to just move it onsite yesterday and the speedometer briefly pinned to 6 o'clock and the came back to normal. But my voltmeter will sweep and then just slowly go back to resting at 0. Not sure if I just need to replace that small side cluster or if there is a wire I can trace and fix for the voltmeter.
Also, my truck used to have ABS but was disabled by the last owner. Not sure if that will have any impact on these issues.
I keep seeing these issues with International. It might be time to find a different truck

 

Update on my '95 9300: I took it to International and they couldn't find anything wrong with the wiring. They said it was the ECM that is the issue, but my mechanic already tested that. International did say that the voltage for the speed sensor wire would be at 0.0 and then jump to 4.0 and then go back to 0.0. It seems that it isn't receiving consistent power. Any thoughts on what that could be caused by?
Also, this is a dump truck and when I wind/unwind my tarp it makes my speedometer move and sometimes max out which cuts power to my engine.