It seems my tach starts acting eratic if it spends to much time at 3500 rpm. If I run at 75 mph for more than a 1/2 hour it will drop down and bounce around 2000 to 2500 rpm. If I get off the freeway and drop my speed down to 60 mph the tach will work fine. Do I have a filter problem?

Thanks

Well........I'm not sure if a '76 has a tach filter or not. If you have a wiring diagram, it should show one.
The problem could be the tach itself(gauge). Could be a little gummy, or sumpthin.
It could be the circuit board(ribbon), on the rear of the tach/speedo panel, or the connection thereof.
It could also be a problem with the ign. module, or coil connection.
I think I would start checking the connection at the dist. cap first, and make sure it is good & tight, with no corrosion, or moisture. Then you might need to pull the dash pad down & check the connections on it.

Hopefully, someone else here will have more info on this particular problem. Adams' Apple38593.8071527778

Thanks Adam,

There is a filter on the back of the intake manifold but I did not see it in my electrical schematic. It seems to act up only after spending 1/2 hour running at 3000 to 3500 rpm. I do want to go through and recheck the wires like you said. I realllly do not want to pull the dash. I did a complete interior rebuild last winter and getting the dash all back to gether was not fun. The problem did happen before the interior rebuild so I don't think that is it.

On a good note, my sunburn from the Skykomish show is gone

If it does indeed have the filter, just unplug it, and see if the problem goes away. I believe it is wired in parallel(?), so the tach should still work with the filter disconnected. If not, you could jumper around the filter.