Had similar prob's with my 84 760 T, never fixed it. ( I know, BIG
help)
But recently, I aquired an 89 780T, when I purchased it I noted to the
seller (among many other items) the gage did not work.
Whereupon he gave the dash a short slap with his hand, right above the
left side of the dash. Instantly, the gas guage sprang to life.
He said, "Oh yeah, common with this vintage".
I now do this every time I get into the carand start her up, and it
always does the trick for mine.
So obviously, there is a connection issue in mine at the dash panel
itself.
It will be an easy fix when I finally get the motivation to take it
apart, maybe a scrub with contact cleaner/copper brush or whatever.
So look at that area, try giving the dash a little spank, see if that
helps.
It's free, and may point you in the right direction quickly if it
causes the guage to start working again.