well, with modern cars troubleshooting over the internet can be nearly impossible. the first/easiest thing would be to get a tuneup, plugs, wires, et cetera. it sounds like it's not getting full combustion, so next i'd get the coil pack checked. this can be done with a multi meter. recently i had to do that to my nephews 98 626. i got the instructions on how to do it by doing a web search for "coil pack test". the only other thing i can think of is it might be the engine computer, but i doubt it.
good luck
mike