Ok, so I seem to be having the same problem, but mine isn't just when I'm at a constant speed, mine happens when shifting from any gear. When I go from park to reverse or drive, the transmission goes into gear like I'm revving up the engine and throwing into gear and when it shifts from first to second it shifts really hard. I bought a bottle of Lucas trans oil and it didn't help. I changed the spark plugs today and the motor idles just fine, but nothing has changed with the shifting. I got on the freeway and after about 20 min, it felt like the trans was slipping or "Surging" like they said. It was like it couldn't make up it'* mind weather it wanted to shift or not, but I was going at a constant speed! I just bought this car on Tuesday and I'm already having problems with it. Any advice and help would be MOST appreciated!!

You could have a bad throttle position sensor. Here'* how to test it..

https://www.gmforum.com/trouble-shooting-test-procedures-141/testing-tps-throttle-position-sensor-279319/

Also, have your car scanned to see if any codes come up. Some transmission codes do not trip a check engine light.

Goatmaster: Sounds like the TPS, but also check your transmission fluid level, color, and smell. I've had both problems, and they have similar symptoms.

Thanks guys! I'll have to find someone with a tester to see if it'* the TPS. I checked the fluid and the level seems fine but the color is more of a clear pink, not red, and the smell is something terrible!! It smells like a gas mixture or something of that nature. I don't know what it'* supposed to smell like, but I'm hoping it'* not like that!

Also, it'* a 2000 SSEi with about 120500 miles. Would you suggest a trans flush or just a drain and filter change? I don't know if the fluids have ever been changed, and I've been told that flushing a trans with that kind of mileage could kill it! I need this car to last as long as possible, but it seems like it'* going to be a money pit and I've had it less than 3 days! LoL!

New transmission fluid is cherry red and smells very similar to fresh motor oil, but you say yours is clear pink and smells like gas?

Touch it and rub some between your fingers; does it feel viscous like engine oil, or somewhat watery?

Ok, I can't really tell what the fluid feels like since I added the bottle of lucas. It doesn't feel like motor oil.......or really any oil for that matter, but it doesn't feel watery either. So, does it sound like the last owner beat the hell out of it and I may need a new tranny? I know I need to change the fluid, but if I flush it, don't I run the risk of killing it that way too?

(I moved this from there into its own thread for better exposure.)

You may have coolant in your transmission due to a broken radiator. You can verify this by taking off your radiator cap; orange-red (or green) water is good, but if you see strawberry milkshake or flecks of red or black oil, you're in trouble.

A radiator swap is easy enough. Unfortunately, coolant can degrade the transmission internals very quickly; if it'* been sitting in there long enough, you could be looking at transmission service down the road.

My advice? If you want to try to save it, perform a tranny fluid/filter change with Lucas, a new radiator, and a good flushing (or three) of the cooling system. You'll find how-tos elsewhere on the forum.

Colin.. water and oil make a milky shake. Trans fluid looks reddish shake in color. OP says the fluid overall looks good.

So I'm looking at spending a lot of money I don't have on this thing? I traded in a perfect 2004 Cavalier for this piece of crap! I took it out last night and the Tach stopped working, and when I looked at how many miles I had driven and that the gas gage hadn't moved I decided to go fill it up. The DIC screen said I had like 306 miles till empty, but when I filled up I put over 11 gallons in! You guys can stop me at any time from listing all the problems! LoL!