Transaxle problems---2000 Malibu V-6

My poor Malli hasn't been right for the last 3000 miles and after $1500 in repairs. I let the dealer change the intake plenum and head gaskets ($900) then my throttle position sensor went bad (that was exciting--engine was stuck at 3500 RPM!! I put it in gear out of curiosity and proceeded to reach 65mph without touching the gas pedal!!) I replaced that myself--very simple. But the original problem I had that got me started learning the mechanics of this car---my transaxle slipping--is still there. There is NOTHING like spending a ton of money on a car and still haveing the SAME problem, it's very deflating.

ANYWAY--ANYONE PLEASE HELP IF YOU KNOW THE ANSWER

When on the highway--60 mph--steady throttle--the RPM will briefly surge from 2000 RPM to 2500 RPM then comes back down, this creates a disturbing "Shudder" in the car. I know it's not the throttle position sensor, but possibly the harness (but there are no computer error codes) I have no way of knowing if the transaxle fluid level is correct (thanks GM!!) because I don't have a lift. But I did have jiffy lube change the fluid at 50,000 miles as recommended for my car (DID THEY CHANGE THE TRANSAXLE FLUID FILTER--I DON'T KNOW) Is my transaxle going bad at 58000 miles?? Wouldn't surprise me I guess, but I am the only owner and I have never dogged my car. Also related--but possibly seperate--the cruise control will shut down during the more severe "surges" AGAIN--there are no error codes---could it possibly be the computer?? I am lost and need help--dealer keeps charging me for "Unable to Duplicate" and I'm still waiting for "assistance" from GMAC.

Any suggestions?

A/T - Shift Flare/DTC's Set/SES Lamp ON
Bulletin No.: 03-07-30-021A
Date: November 19, 2003
TECHNICAL
Subject:
Neutral Flare and/or RPM Flare While In Drive, No 1-2 Upshift, Service Engine Soon (SES) Light Illuminated, Diagnostic Trouble Codes (DTCs) P1810, DTC P1815 Set (Reprogram the Powertrain Control Module (PCM), Replace Transmission Fluid Pressure (TFP) Manual Valve Position Switch)
Models:
1995-2003 Chevrolet Cavalier
1997-2003 Chevrolet Malibu
1999-2003 Oldsmobile Alero
1995-2003 Pontiac Sunfire
1998-2003 Pontiac Grand Am
with 4T40E Transmission (RPO MN4) or 4T45E Transmission (RPO MN5)
Supercede:
This bulletin is being revised to add reprogramming of the PCM on 2003 V6 models. Please discard Corporate Bulletin Number 03-07-30-021 (Section 07-Transmission/Transaxle)
Condition
Some customers may comment on a neutral flare and/or RPM increase while in drive or no 1-2 upshift and/or the Service Engine Soon (SES) telltale may be illuminated. On 1995-2002 model vehicles, the Powertrain Control Module (PCM) may set a DTC P1810 while on 2003 model vehicles, the PCM may set a DTC P1815.
Cause
The cause may be the transmission fluid pressure (TFP) switch (also known as the pressure switch manifold (PSM)). It will be referred to as the TFP switch in this bulletin.

HOWEVER The transmission needs to be removed from the vehicle in order to replace the part, which is why I myself did not have done.

For now, I downshift to 3 if going uphill at highway speed, or if slowing down signficantly but do not come to complete stop, I then downshift to 2. After downshifting, then upshift until I get to the speed I want and eventually make sure that I am in Overdrive again.