I've got a 96 with the 4.3 motor and I'm working through various issues with the truck as it is new to me. It has only 82,000 miles, but it has not been well maintained. The persistent issue that does not seem to be going away is the tach bouncing from about 1, 500 to 1,200 rpm when I'm traveling about 40 mph. It only seems to happen if I'm holding a constant speed and constant rps. If I'm accelerating, there is no issue. It only seems to be if the speed and rpm are constant for at least several seconds. I do not hear the motor changing pitch and I do not notice a change in speed. In my attempts to resolve a number of other issues, I've replaced the TPS, rotor, distributor cap, plugs and ICM. My testing of the fuel pump showed a slow leakdown from 55psi, but given that the speed of the degradation changed depending on which tester I used, I'm not confident that I have a leak down problem at all because the car starts right up regardless as to whether you let the fuel pump prime the system. There was evidence, however, of a leak upon removal of the upper plenum.

So, I'm at the point that I believe that it could be a VSS or still a fuel issue. I have no issue with the speedometer bouncing, so I'm thinking that is an unlikely source of the problem. It has the CPI injector system on it still. I had the manifold off because I saw some clean spots near on injector #1, but I concluded it was the fuel pressure regulator and changed that out since there were more clean spots under the FPR. It is possible that it wasn't the FPR (as the fuel pressure leakdown test results haven't changed after changing the FPR) and that the clean spots from leaking fuel were from injector #1 and that gravity carried the fuel back to the area under the regulator.

I had the instrument cluster out earlier thinking it was a solder joint somewhere. I touched up all the joints around the tach, but nothing changed. Interestingly, this instrument cluster had been worked on in 1996 as there is a sticker on it and flexible trace points are soldered on where the pins meet. I looked at another in the junkyard and it was clean of solder in these spots.

The rpms always go down per the tach. I thought it was a ground issue for a while, but I've cleaned the grounds (except for the ones on the heads, which I cannot even find due to clearance issues).

I'm open to suggestions. I'm kind of thinking its fuel and that the hose at injector #1 leaks sometimes as the shiny part of the manifold was the top half of where the injector plugged into the lower intake (and then along the side of the lower intake near the gasket sealing surface back to the FPR). This would account for rpms going down (fuel pressure being lost) and since its only a couple hundred rps, that might be why i don't notice any in the engine. Of course, this really doesn't explain why the issue comes up at only certain rpms and certain speeds.

Thanks in advance.

 

It's almost certainly the TCC engaging and disengaging. It's normal, unless it is doing this rapidly.

 

I'm not an expert like Lesmyer here and he probably is right with the answer here.
I would just add 3 cents from my side - once You start accelerating, the fuel demand gets higher. That usually means the problem would appear an opposite way - once You start accelerating, not on a constant speed.