I have 1999 pontiac grand prix GTP, in the morning when the car is cold it will perform fine, until it warms up and than when accelarating it is very slugish, however if I maintaine same speed or increase speed ever so slightly it will run smooth and also it idle great. any ideas?

When was the last time that the fuel filter was replaced?
Have you had the pressure of the fuel pump checked?

A fuel filter that becomes progressively more and more obstructed with debris will strain a fuel pump to the point of failure.

never had replaced, dont you think it might be plugs/wires ir fuel injections?

Perhaps it might be badly fouled spark plugs.
When was the last time that you replaced the spark plugs?

But, then again, on a car that is at least 9 years old, if it has not been maintained properly the possibilities are numerous.

I would start by bringing the car up to date on any and all of the maintenance procedures that you might have skipped over the past 9 years.

I think you are right, thx