On my 1999 GMC Jimmy 4X4 V6, when driving at 80 KM/HR, I notice the tachometer needle drift up and down 2000 - 2200 rpm, even though I'm holding the speed/gas at a constant rate. It does this whether or not the A/C is on. It doesn't seem to have any problems at higher speeds of 100 - 110 KM/HR. It shifts fine, no slipping and it's relatively smooth.
I thought there could be a problem with the overdrive on the transmission, but wouldn't venture a guess without being able to get a complete analysis done. In your experience, do you think the problem is related to the transmission, or perhaps a computer/sensor controlling the transmission, fuel injector, or something else?
My OBD2 Reader doesn't say anything is wrong yet.
Thanks for the help

 

The speed you mentioned is right around the typical torque converter clutch lockup speed and the RPM fluctuation is consistent with torque converter slip.

The problem is likely in the transmission, but without further hands on diagnosis, I can't really speculate further.

 

Thanks, I suspected the same