This might be a stretch I know.
I have an LS1/TH350 in an 84.

Has all autometer gauges, this is the speedometer
https://www.autometer.com/3-3-8-spee...E&gclsrc=aw.ds

I bought the car this way from a friend's dad who said last he remembered speedometer worked normally so I'm going in kind of blind here.
Speedometer will work the first few minutes after starting the car but then quits. It won't work again until car has sat for like a half hour. I'll have the car up on stands this weekend so I figure I'll check at the speed sensor itself? Any other idea where to look first? I've never messed with third gen gauges let alone an autometer speedo