2000 Pontiac Grand Prix transmission problem?

I'm assuming that you already have, but check the transmission fluid level and also look at the color of the fluid (supposed to be red).

Is the check engin elight on?

How does it fell as it shifts through the gears?

Does the transmission have to upshift after you've let off the gas, coast for a while and then reapply to gas?

You may have to have the pressure read at a transmission shop. It kind of sounds like once you let off the gas it disengaged, yet doesn't downshift.

Please excuse the fact that I didn't explain "checking the trasmissions' pressure" when I first mentioned it.

The automatic transmission, in some cases shifts by using hydraulic pressure (pressure built by its hydraulic pump), and in all cases these transmissions use this hydraulic pressure to push its clutches together, providing enough friction to keep them togehter as they spin inside of the transmission. This in turn, turns a shaft which eventually leads to turning the drive axles of the vehicle, causing it to move under its own power.
As the transmission shifts, each gear requires a certain amount of pressure in order to operate properly (the pressure required is usually different for each gear).

By checking the fluids' hydraulic pressure the technician can determine if the clutches inside of the transmission are capable of providing enough friction to move the vehicle properly (without slipping or other shifting problems).
Once this is determined they can then make an informed decision on the best repair for your problem.

Also, if the fluid is brown, instead of red, this means that the friction material from the clutches is falling off. This material gets into the fluid and changes its color; which is a indication that the clutches are wearing out (which greatly reduces their ability to produce enough friction to shift the transmission properly).

Often a last effort (before having the transmission overhauled) is to change the transmission fluid and filter; to see if that remedies the problem. If so, you've saved a lot of money. If not, you haven't lost a lot of money (at least that used to be the case, but have you seen the cost of automatic transmission fluid lately?).

Call around to some of the transmission shops in your area, there may be one which will check you transmission for FREE (they willsometimes do free inspections to drum-up business). However, don't buy anything until you're told me what the results are. There may be a more economical way to handle this than a complete overhaul.