I just got this GP in the beginning of feb. Its a top swapped 99 SE with a 3.4 pulley/ 1.9 rockers/ TOGs+ 3" downpipe and catback/ zzp crap tune.
From what the PO told me, the tranny in it is original from his knowledge. He said he installed a new valve body in it back in '04. The car has 232k on the odo now
Anyways, I have to admit that I've been kind of hard on it since I bought it. Today was the first time it slipped when I hit WOT while merging, and lately its been hesitating for a second to upshift.
Replace filter+ drain fluid and re-fill.
Or
2000 Buick Century Automatic Transmission
If he can do $300 I'd strongly consider it. From what I read, Centuries came with the 4t65e I haven't had a chance to check the tranny fluid yet but I'll see how that looks.