[jeffcoslacker]

Well, to get the best result at the mechanic (and hopefully a correct repair that you'll be satisfied with), you need first to be able to adequately describe the problem.

Things we need to know include:

How often?
Predictable or Random?
Occurs cold, hot, only after warm up, in certain weather conditions, etc.
What speed?
Accelerating, cruising, stopping
Any noises or smells associated with the problem?

In your case, does the motor seem to run normally, without any misfire (stumbling, cutting out), and just seems flat, like it's towing something heavy, or does it seem like it isn't accepting the throttle you are giving it?

Just a wild guess I would think if you are referring to when you try to pick up speed when already moving at a decent speed, like above 40 mph, then you may have the issue where the torque convertor lock isn't releasing on demand all the time.

If you are talking about acceleration from a stop, it could be anything, more info might let us point you in the right direction.

But in general, the more info you can provide, the quicker (and cheaper) the diagnosis will be when you take it in. When the problem occurs, note everything you can about conditions, how far you drove before it started happening, is the motor fully warmed up, how fast are you going, what gear, what engine speed, uphill, downhill, flat, hot/cold outside, rainy, dry, etc. All this can be important.