Does the speedometer needle have any bounce to it? VSS can also cause weird issues. Other thing to check is make certain the engine ECU under the bonnet isn't corroded.

 

I’ve had someone else tell me that their ECU was the issue & when they experienced what mine was doing said that they thought it was the ECU. I’ve checked it & didn’t see anything corroded. Is there any other way to check if it’s working correctly? Also my speedometer needle never has any bounce, just the RPMs.

 

My speedo is nice and smooth too. Here's my latest.

When I first bought the truck, 8 years ago now, I did a lot of service, that included changing the tranny filter and adding a bottle of Lucas Transmission Fix (Disco Mike's favorite) very thick stuff with the balance Valvoline DEX/MERC. So I got thinking about pressures and viscosity. Now I have done two drain and fills (about 4.5 quarts each time). It's still bucking, only if upshift to 4th occurs before 50 mph occasionally, which is to say sometimes it's fine even before 50. After 50, nice and smooth. Earlier today, I added a bottle of Sea Foam Trans Tune, very thin stuff. I'll drive for half a tank and see how it performs, and get back (to where I once belonged...)

 

3 days of driving around with a bottle of Sea Foam Trans Tune in it (city and highway, about 150 miles). Actually about 13 - 14 ounces in the tranny (I put the rest in the power steering reservoir). There have been no unstable shifts, and I tried all the usual scenarios. Now it only tries to shift after 50 mph, not after 45.

That little bottle seems to be magic. We'll see how it goes longer term. I think I'll do one more drain and fill. (Get back Loretta...)