It's probably engine-related. A friend of mine recently was having the same shifting problems in his 93 Buick, and it turned out that the engine needed quite a bit of tune-up work, including two new transformers for the distributor-less ignition system.

If you haven't done a tune-up recently, replace all the usual stuff...spark plugs, plug wires, cap/rotor (or related spark timing components, depending on what your car has), and possibly the air filter. Also, check the timing with a timing light, that could also be causing power loss if it's off.