1995 Trooper shifting problem.
 

I have an intermittent shifting problem. When driving normal at the speed you would expect to shift up, sometimes it won't shift into the next gear. If I step on the gas the rpm's won't increase, speed stays the same. If I let up easy, it will shift up. Does anyone know what's going on? I'm a little worried about having it flushed. Sounds like that can cause problems.