I had the same problem with mine...turned out to be plugs and the catalytic converter.
I use Jose Duran. He is at 35/Walnut Hill area. 214.351.4254.
He has been great with my car and MUCH less expensive than any place I've found.
88 BMW E32 735i (make it go away)
95 Volvo 960 "Inga" (valve problems)
95 Nissan Maxima (the indestructible)
89 Ford Ranger XLT (ugliest truck in the South)