You could try changing the transmission filter and the fluid.
i have the same problem. do not waist any money on sensors, i have replaced all the sensors and wires, what it is, is the thing that shifts your transmission, your car is taking off in high gear, my car takes off really slow which is a 1992 Pontiac Bonneville SE, and do not drive it until it is fixed, or you will end up putting a new transmission in to it.
2000 PONTIAC BONNEVILLE 3.8L 6-cyl Engine Code 12000 PONTIAC BONNEVILLE 3.8L 6-cyl Engine Code KCAPACITIESEngine, with filter..........4.6 quarts [1]Wix Oil Filter...........51040Cooling System, Initial Fill..........10.0 quartsAutomatic Transmission, 4T65E Initial Fill..........7.4 quarts [2]Automatic Transmission, Total Fill4 speed 4T65E..........13.0 quarts1. After refill check oil level.2. With the ENG at operating TEMP, shift TRANS through all gears. Let ENG slow idle in PARK for at least 3 minutes. Check fluid level and add fluid as needed. Do not overfill.10W-30 is the preferred Oil for both.
This controls how quickly your automatic transmission shifts to the next gear. The ECT is normally programmed to shift for fuel economy, resulting in slow acceleration. Putting it into power mode shifts gear for better acceleration while sacrificing fuel economy.
I would first check the u-joints in the driveshaft. But it sounds like the transmission rear mount is broken. CHECK TRANSMISSION MOUNT.
It should only do it under braking. It is to help slow you down.
balls. balls will slow shafting
An automatic transmission might be slow shifting from first to second because of low transmission fluid. You might also have damaged the gears in the transmission at the area between first and second.
Very slow or fast transmission speeds become from what combinations of hardware and software?
Try making sure your transmission is not overheating, as well as check the fluid inside it as it could just be low or needs changing.
I've been using it exclusively in mine (all 1995 and older). Use of a transmission cooler if needed since it doesn't like the heat. An awesome upgrade if you like real positive shifts! I've heard it may cause p0740 codes in 96+, but a 50/50 mix of dex3 and type f should be okay if you have a slippy 96+ that you are trying to avoid a rebuild on... also have the shift solenoids pulled and cleaned - as blocked screens=poor fluid flow=slow shifts=burnt clutches.... Cheers!
you might need a new one soon. but first check your transmission fluid.
egr valve