It could be a bad shift solenoid, I took a quick look at Auto Trans diagnostics in my FSM, Ceck the revolution or speed sensor curciut. if thats not it check The Throttle position sensor. It could also be the trans fluid condition and the shift solenoid valve, shift valve Pilot valve and and pilot filter.

I downloaded the FSM in pdf file form from this site,
Free Car Service Manuals

I hope this helps you out.