I'm thinking it's something in the trans. My '04 Monte and the wife's '05 Impala both do that (from a dead stop, you go to accelerate and sometimes it feels like a hard shift). Neither car does it a lot (once in a week or two).

 

I have the same issue with my 04 intimidator and i tried to find some info but no luck just like you guys it happens once in a great while, sometimes 2-3 times a day, then it will be fine for a while i have no idea whats causing it but i plan to have the trans rebuilt this winter with a shift kit and new torque con. and hope it goes away, still debating on using a transgo shift kit vs. a ZZP one any thoughts?
another thing it dose is if i put it into neutral and i am still moving and then put it into drive (like when ya rev it while rolling) and hit the gas it will bog way down for a few seconds like its in too high a gear then it feel like it kicks down and go and it will do that every time, not that i do it alot but i did try it a few times after i noticed it and it did it even on days that it never did the hard shift thing

 

i have a transgo. it isnt really harsh just quick firm shifts.

 

I am just curious. Have you checked your tranny fluid? What could be happening is that it has eought on flat ground but up or down hills causes issues. I had some problems with my Monte when I first got it because someone didn't use a tranny pan gasket (rtv) and it was leaking so I could fill it up and be fine for afew months then getting on or off the highway it would have problems shifting and it was low....so since it something that is easy to check first....figured I would throw it out there.

 

So thought I'd post an update here. No recurrence of the issue! It seemed to go away and hasn't been back since. I never took it over to have it checked since the issue didn't come back. I figured I may just be looking for problems at that point, so even though I was on my way to the garage I turned around and came home! I'm hoping to get through the winter at least if not more with the Monte. Have my eye on a nice 2012 CTS Coupe, but can't pull the trigger on that just now with Impala and 2500HD payments. I'm still convinced I can get the odometer to 300,000 miles - at 218,000 now, so I've got a ways to go assuming nothing major breaks.