possible tranny problem?

ive got a 96 silverado excab 4x4 w/ about 180k. i just had a used tranny put in a few thousand miles ago with 80k. now when driving steady at ~60-80 mph the rpms seam to drop quickly once in a while, maybe every 5-10 minutes. i cant tell if its the tranny or not. its like a quick jerk rpms go down 2-300, but it drives fine besides that. i cant figure it????

Xr,
I could be wrong but I believe that is normal. This is a locking torque convertor. You should be ok once it locks up it will almost act lika an additional shift.

i have a very similar problem. when i'm cruising 55-70 mph it seems at a certain place on the throttle the rpms will go up and down as long as i hold the throttle there. if i accelerate or slow down it will go away. i thought it was a tranny problem. now i dont know?

Transmission fluid level up good?
The lockup is turning off and on.
Make sure you are holing the gas pedal steady.
A engine miss or a lose wire on the transmission or a bad Brake light switch or eve a TPS sensor will do it.
I would suggest a engine /transmission capable scanner and test drive with scanner hooked up.
Watch tps sensor reading and brake on and off and tcc command readings.
And a good code check.
If any codes post back no.
MT

I have the same problem. While driving over 30-35 at steady speed the truck seems to slightly surge. It is not enough to show on the tach but you can feel it. It started doing it around 120K. I can't figure it out. It also seems to get lost between overdrive and 3rd gear. While driving at a constant speed the motor starts lugging and then the truck starts to jerk. If you give it a little gas it just gets worse. But if you give it just enough it takes the lower gear and is fine for 500 feet then does it again.
I have completely serviced the trans, changed plug wires, fuel pump, pressure regulator, fuel filter, plugs, u joints. Checked for codes and there are none. No miss codes nothing. The truck is now occasionally backfireing or poping through the top end when you floor it around 4500 rpm. It is driving me nuts.
Any ideas????????
Thanks

Welcome to AF
For a problem on yours start a new post/thread.
And give all info like year make and model and engine transmission and mileage.
But I would not be winding it 4500 RPM sitting still