Hard noise when shifting gears

Lately I'm experiencing ruff shifting and some very un pleasing noise(kind of a grinding noise) when i shift from first to second gear, especially when engaging the first gear and realising the clutch solely. The problem appear only when the engine reach full operating temp like in stop and go traffic or after 15-20 min of driving. When the i start the car and the engine is not "hot" the problem dose not exist but as the car become hotter i need to press the clutch harder and harder to keep the gears shifting smooth. I replace the stock clutch to a centerforce unit and change the hydraulic fluid about 8000 mile ago and was very happy with the result until this problem start about a mon' ago. I bleed to the clutch fluids again when i first notice the noise and check for leaks in the lines but every thing looks fine and the hydraulic fluids R full. I was thinking that the heat from the headers maybe effecting the hydraulic line when the engine heats up? Is there a connection between those symptoms and the "drill mod" i was reading about in this forum? any help/suggestion will be greatly appreciated, Thanks guys.