1998 C1500 5.7L 4L60E transmission shifts very hard after stop and go driving. Only happens in summertime when temps are high, no problem any other time, but. Also some slippage at moderate speed when letting off the gas and then accelerating. Also some slippage while cruising at times, not necessarily at the same time as hard shifting. Is my transmission on it's way out? I've done a drain and fill a couple of times, seems to help temporarily but doesn't last.