I have a 2000 blazer 4wd that surges as it changes certain gears in a a\t. Mainly at speeds of 35 to 40 and 55 to 60 it does it the most. It does it more when the ac is on. I think it is a vacuum problem ,but can't find any leaks. The rpms's will surge up and down, and it seems to be when the gears are switching, but also sometimes at a set speed. I have already changed the oxygen sensors, and it has a new catalytic converter.

 

Go to the main forum page and in the goggle box search something like "Transmission Surge Vac".
Codes? No codes?

 

It is showing no code, and I changed the throttle sensor just to see if it helps ,and it didn't.

 

I'm having the same problem as you. Been doing it ever since I bought it. It's gotten worse now and is much more frequent with the A/C on. Got a code on it 2 days ago.P1810 "Trans Pressure Switch Manifold Fault". Just gotten around to searching on here and found this post. I'll let you know what I find out.

 

Just a shot in the dark, but maybe its something to do with your torque converter. You said it does it most at or around 55 which is right around where your torque converter unlocks.
Try driving at about 60, when you know your in overdrive, try barely tapping on the brake. Your RPM's should jump a few hundred. IF it jumps like that, its likely not something to do with your TC.
This prolly wont help you, but like I said its a shot in the dark..

 

my check engine light finally came on.I"M going tomorrow to have it coded to see what it comes up with. It now does it at any speed where i have to hold the pedal at a steady speed. But I will try anything to see. thanks for the advice.

 

Let me know, I'm interested in what you find out.

 

the code came back as tp sensor throttle position sensor. which I just replaced just a couple of days ago. I could of gotten a bad one so i'll try it again.