i have a 99 tahoe and the trany started shifting like it has a shift kit in it. the ck engine light comes on and auto zone says the code is 304 and its the trans control module. also when i stop or take off i can hear a clunk and feel some thing it doesn't do it every time but it shifts hard after five or ten min. of driving. any advise would help me out.

 

Never heard of a 304 code. Maybe a P0304 - Cylinder 4 Misfire Detected.
Trans codes for that year usually start with P0.... For example P0757. It is possible that what ever code you are getting is causing the computer to make the trans have max line pressure. Thus the hard shifting. Vince

 

On a Vortec 350/4L60-E Tahoe there is no "trans control module".
A code that would make more sense would be P1870. Like Vince says, the PCM is programmed to command maximum line pressure when component slippage is detected.

If the transmission and valvebody in the Tahoe are the original I would bet that the torque converter clutch regulator valve bore is wore out and allowing fluid bypass. Slippage is being detected in the torque converter clutch. The reason you only get it after driving for a while may have to do with the roads you're driving on. The torque converter won't lock up long enough to detect slippage on city streets. When that kind of slippage starts to happen it's usually on a highway where you're cruising along normally and begin to go up a grade where you ease into the throttle just enough get more engine torque but not enough to invoke a downshift or converter disengagement. Under these conditions torque is high and line pressure is relatively low.

Also, the PCM is programmed in our trucks to try to lock the converter even during wide-open-throttle accelleration. If it's the regulator valve causing the problem you probably won't see it doing this because line pressure is high.