81 308 RPM Erratic

Hi everyone just started to notice a problem and having a hard time trying to figure out what it could be. About two weeks ago I noticed that the RPM needle started to jump up and down a bit at about 2500 RPM and up and could feel it in the seat that the engine was running with a slight hic-cup leading me to believe that the RPM needle was reading correct. So I thought it might be the gas I was using (Shell here in Denver) so when it was about empty I put in Conoco and a bottle of Chevron fuel injector cleaner and still having the same problem. OK this is what's confusing me to, when in neutral and I rev it through the RPM range and hold it steady at each 500 RPM increments and up to 5K starting at 2k it's smooth, idles smooth too. Any help here would be great, thanks.

 

Peter you are awesome! It was the digiplex grounds; I can't believe it was that simply. I had no idea those boxes were under there, I would have never figured it out without this web site and your help. I cleaned both boxes (801's) and re-grounded both wires. Both ground wires were on the same bolt so I separated them, each one to a separate ground point. Works like new hope it last a while. Thanks again.

Pauly