2003 Chevy Silverado Transmission Woes

Two questions here:
1. My transmission slips when I apply a load from the engine. I have a dual friction clutch and have replaced the slave cylinder 3x as well as the master twice. The slave cyclinder seems to remedy it for a while, but as time progresses it goes right back to slipping again. All this began at about 22K. It seems to me that when applying a load, heat builds up and causes the slave to build pressure, thus failing. Almost as if it is bypassing fluid inside. After the slippage, the clutch pedal is very hard. Does my diagnosis sound true?
2. Is there any after warket slave cyclinders out there besides what GM has to offer?