Gear engaged but car doesn't move

I'm working with some family members who have a 97 Bonneville with the 3800 supercharged and approaching 300,000 miles. It already has a rebuilt engine and transmission.

Just recently when putting the car in gear it sounds like it goes in gear (the engine rpms settle down) but the car doesn't move. Under the hood I can see the arm that shifts the transmission. It moves normally and appears to engage the transmission properly, but the car just doesn't move.

It doesn't do this all the time, but it does it about 90% of the time. The other 10% it goes it gear and drives just fine.

Today while testing a little more we put it in drive and it didn't want to go, but with the RPMs up around d 3,000 the car creeped forward a couple feet and stopped.

Anybody have any ideas what might be causing this or what we should be checking? Any help would be greatly appreciated.

Anybody??? Could it be an issue with getting the right vacuum pressure to the transmission?

I am by no means a transmission guy, but I highly doubt it's a vacuum problem. More likely something in the valve body, or deeper. You said the trans was rebuilt, was it done recently? What does the trans fluid look and smell like? Is it red or brown and does it smell burned?

Yeah, the first thing to do is check fluid level and condition......

Then check for codes......

Certainly sounds like an internal problem.....about the only thing you can do, is drop the pan, and if you find metal, uh-oh....