1994 Saab 900s transmission?

I recently bought a 1994 Saab 900s. It's a '94 w/ about 135K miles. I rarely have this issues but I was looking for comments on it. Once in a while the car "bucks" when entering a highway, off the ramp. I was wondering if you though fuel filter or tranny or what? Car seems to shift fine run fine all the time except at these times. It seems to bee accelerating, up hill like when you merge onto a highway. The car did sit a little bit before I bought it. About 3 weeks. Thanks for any suggestions.