My 98 Z-34 had a bad whinning noise, the transmission was shifting slightly hard, and when i gased it hard from a dead stop, the engine reved before it caught a gear. I had my transmission serviced and it worked great afterwards for only about 2000 miles. Now it is back to the same characteristics, only now it seems to be losing a little power. Does anyone have a suggestion? Can anyone tell me why the service helped but only temporarily?

you probably have a faulty PCS solenoid, fairly common issue on the GM 4t65e transaxles.