First thing is to check the basics-fluid level and condition of the fluid. Sounds like the internal clutch bands maybe worn. If the fluid condition is less than desireable, there could be a problem with parts sticking in the valve body also. You could try to flush the system, but sometimes that makes things worse. I would take the vehicle to your repair shop for diagnosis. I would let them verify the problem and go from there. If it is a trans related problem, chances are they will tell you the trans will need replaced or rebuilt. How many miles are on the vehicle? When was the last time the vehicle was serviced? These are just suggestions | 
