Generally in all automatic cars, when you let off the gas pedal, the torque conveter disengauges to help with fuel economy. When you press on the gas again, it has to reengage. Sometimes, however, it does not engage properly, and could be the root of the sympthom you are describing.
Generally, the first thing I'd do is take it to a place like AAMCO maybe or and independant tranny shop and have em flush the system. Spending $80 on a trnny flush is better than speding $1100 for a rebuild on the tranny. If the flush helps or solves the problem, then thats that.
If that doesn't fix it, then the second thing I'd do and what thye'd prob do at the time of the flush is hook it up to a diagnostic computer. Also, I'd get the auto tranny computer either reset or reprogramed.
If they can figure out whats happening from the diag, then you may have to opt for the exspense of getting the tranny rebuilt. Often, when a auto tranny starts to slip, it is the torque converter seal that deteriorates. When this seals goes, your tranny slips because fluid is going where its not supposed to go. If it goes on too long, this could damage the torque converter, and you'd need to replace the torque converter in addition to the seal. Sometimes you can get away with just the seal, if the converter is still good. That is generally left up to the technichian to decide.
Hope that helped some.