repaced the clutch and pressure plate

I have a 93 madza mx3 stickshift, I was advised by my question last time to take it back to the mechanic, that the clutch needed to be replaced. I took it back (they put a new clutch in it last year.) They pulled it and repaced the clutch and pressure plate. Saying it was still under warrenty.It's hydrallic. I'm still having the same problems. They tolled my car back to the shop and are trying to figure it out, do you have any ideals? When I drive it for 30 minutes or longer it no longer will go forward in any gear and the rpm's rev up real high, there's no power. It always happens when I'm going up a hill and then it just won't go. It will shift into into gear but's there's nothing there, it's like nutral in every gear. Help, they say they can't figure it out, they gave it a tune up 2 weeks ago also. Thank You,
Sheryl