My2008 Enclave accelerates nicely. when it reaches the speed at which the transmission switches to the next gear (20-30 mph) the car shakes and vibrates and the rpms fall.

Our vehicle does the same thing. Did you ever find someone who was able to properly diagnos and fix?

I have the exact same problem

I also have a 2008 Buick Enclave and when I reach 40 and then when it hits 50 and then 60 nobody can seem to find the problem I got the throttle changed prior to this happening

We were told it may be related to a motor mount which needed replacement but this didn’t resolve the issue either so still a problem

Same problem I’m having

Anybody found an issue to this problem???

Same issues here too. Did tire alignment & changed transmission fluid. No change. Ugh

our 2008 started doing this not too long ago, at first i thought it was maybe a missfire. Being a mechanic i hooked it up to a computer and checked to see if anything would throw a code. turns out that the input sensor from the engine to the transmission was going bad. i though to myself, thank goodness i dont have to replace the transmission. so i changed it, and it helped the problem slightly, still a slight shutter. so doing more digging i found that these transmissions have a bad habbit of going out, and that it is only a matter of time before i have to put a new one in it. its a common problem in the 2008 - 2010 models. sure i could just put about 3k into a new one and keep it, but something you should alsoknow is that at around 130000k miles the timing chains will eventually mess up. i am looking to get rid of mine now to avoid future problems. hope something from this was helpful

All the above is true. 2008's were the worst. GM did a lot of warranty work on these units. I believed they replaced the wave plates or something like that. The repair is 3G$ as the mechanics have to remove the old trannny, repair it and then re-install. A lot of labour hrs. I have a 2009 with 222,000 km's.I nurse mine through the 5th and 6th overdrive gear and it is still working OK for me.

I had this same issue. In the past I had a throttle body issue and a good cleaning helped it. Now, it is doing it again. I just got a new transmission and I have little confidence that it was done correctly since the mechanic had it for over a year, he cross threaded my axel, cut my wheel speed sensors and said he drove it over 100 miles. Yet, the minute I got on the interstate, it my car jersey to the middle lane from the on ramp. I am in the process of trying to figure out if the shudder/sputtering/slight loss of power is related to my car sitting for over a year or other unrelated issues. Ironically, it acts just like it did when I cleaned the throttle body and it was fixed.