I have a 92 Bonneville and think I may need work on my transmission not too sure... the car on the regular road drives fine for about the first hour, after that time the car will hesitate for a second before taking off and all gears will shift fine after that... Also when driving on the highway the car will shift normally but is i drive over 70-80 mph for about 10 mins the car will shift to overdrive and stay there... I see that the RPM are going up and keeps going up when i press the gas, the only way for the car to shift back into the last gear is if i slow down to about 55 mph and then the cycle continues when i accelerate to 70 mph again... then when i get back to normal roads it will hesitate for a sec then go... i always make sure that the transmission fluid is full when i am driving... if i let the car cool down it will not do this right away ... this has been confusing me for a while and anyone i tell... can someone please help me with this?

So are you saying that when you first notice things after cruising at 70-80mph for 10 minutes, that you will see the rpms increase with or without an increase in velocity?
What does your tranny fluid look like? Pink? Brown?
Last fluid change? Filter change?
How many miles on that transmission?

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1996 Bonneville SSEi
- Stock Is A Bad Word -
3.6, FWI, 1 Range Cooler, and Energy Endlinks couldn't prevent a spun bearing.

not to sure what you mean by increase in velocity, but when it shifts i do feel the car jerk a little bit and when i press the gas it is harder than normal. Fluid is pink, i had a leak with the transmission fluid after putting a couple bottles of lucas in it has stopped leaking. Have not changed the transmission filter. did not know there is one.. I am a beginner when it comes to mechanical issues, i usually take it to the mechanic but that is getting too costly and finding a good one is troublesome, so i am trying to fix it up myself. the car mileage is 251K

In addition to the questions from Mike, after a drive and you're good and warmed up, pull the trans dipstick out. What does it look and smell like?

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93 SSEi, 95 SLE (supercharged) 97 Buick LeSabre Limited
PontiacDad at WCBF '04: Cruise control? That's like surrendering!
Comprehensive guide to troubleshooting, rebuilding, and modifying Eaton Superchargers

Just got back from taking a 30 min cruise in the car. After i got home i pulled the dipstick out and the fluid was full and color is pink. also the fluid did not have any smell to it, smelled like regular transmission fluid. No burning smell

Well here's my theory, you can take it for what it's worth.

Sounds like on the highway your torque converter isn't locking. I know that on the 4T60Es the PCM won't allow the TC to lock if it is detecting too much slip, which may be what is happening in your case. Could be simply that you've got a lot of clutch wear, which certainly isn't surprising at 251K miles, and it may be time for a rebuild.

That's my theory anyway, I'm sure others will pop in.
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-Dan
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That's where I'm leaning right now too, but before you go that route, drop the pan and change the 7 quarts that dump out. It may buy you some time, it may solve the problem, and it may make no difference.

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Click here for mod list for both cars
93 SSEi, 95 SLE (supercharged) 97 Buick LeSabre Limited
PontiacDad at WCBF '04: Cruise control? That's like surrendering!
Comprehensive guide to troubleshooting, rebuilding, and modifying Eaton Superchargers

Some have said that the filter in this trans is nothing more than a Teflon screen... Thats not the case, its actually a fibrous material meant to filter out clutch material and various particles... If the filter get plugged it can cause some of the issues you are having...

Its also very possible that you could have a weak pump or a bad valve body gasket... Hard to say..

Might have too much Lucas in it too... Too thick is no good..

Change the fluid, filter and pan gasket first then we will go from there..