1999 Pontiac Grand Am Acceleration Problems

Pontiac Grand AM GT

Problem:

Whenever I push down my gas peddle all the way, the car stops accelerating. This problem occurs at any speed whether on the highway or in town. However, if I accelerate by pushing my peddle down only part way, I have no problem. I've had this problem for over 1.5 yrs. Now and the fixes I've gotten haven't worked. The "fixes" over the last 1.5 years include:

-Complete tune up, catalytic convertor, oxygen sensor. 2 weeks ago, my car broke down twice b/c of a bad alternator and battery, I got both replaced. Again, I've had the acceleration problem for over 1.5 yrs, but it's never been this bad. It got much worse it seems after I got my alternator and battery replaced (but I don't see any connection).

-More info: When I push the peddle, the engine can rev up to over 4,000 rpms, but it slowly rumbles down if I push the peddle down all the way, and as it rumbles down it stops gaining any speed (whether I'm going 30mph, or 70mph, it doesn't matter.). Also, the gears shift smoothly and the check engine light is not on and hasn't been on. Also, when accelerating with the gas peddle down maybe 1/2 of the way, the engine smoothly accelerates past 2000 rpms smoothly but around 3000rpms, it seems to struggle a few times and then continue to go to higher rpms. Also, the car revs fine in neutral.

-Update: Service Engine Soon Light came on today but then went off after an hour of operation.

Any help would be great

Thanks !