Problem with my '99 Pontiac Firebird ?

So are you going to take your car out drinking for its 21st birthday?

Anyway, it can't be the throttle position sensor.  The only way RPMs can jump in a standard transmission that is engaged is if some element in the drive-train slips.  Anything wrong with the throttle causing surging would be no different than suddenly adding gas when the transmission is engaged, meaning any increase in acceleration would be matched by a commensurate increase in actual speed, not just a jump in RPMs. 

It has to be something along the drive-train between the camshaft and the wheels that's slipping. The jerking or lurching is likely caused by it first slipping and then starting to catch again and then slipping again, the jumps in RPMs being seen when it's slipping.  That behavior discontinuing when you shift means that shifting somehow causes what's slipping to reengage.  If it's always the same gear, it has to be your transmission, despite you saying it isn't.  If it's not always the same gear, maybe it's your clutch.

Have you checked the clutch yet? After it's warmed up, at a dead stop, put the tranny in 4th gear. Rev the motor to 2,500 to 3,000 rpm and then release the clutch pedal suddenly. If the rpm's slowly drop and the car barely moves the clutch is slipping and needs replacing. If the motor stalls the clutch is good.

It's likely the clutch. Nothing else would let the rpm's "kick up really high" while in gear.