2000 Hyundai Elantra the clutch started to slip

I have a 2000 Hyundai Elantra with 150000 miles on it. I left work one day and got on it pretty hard and shifted into second and the clutch started to slip. I assumed the clutch was going out since the car has never had one put in. Took it to a local mechanic and it was just the slave cylinder. Clutch worked great for about three weeks then started acting up again. My dad and I bled the system and it fixed it again for about another three weeks. It will not slip in first gear but it will in the rest of them. It is not consistent in the slipping either. Under easy driving conditions there are no problems, but if I try to pass anybody and shift with the rpm's up it will start to act up again. I figure if the clutch is going out it would slip consistently and steadily get worse which is not the case. I would like to know if its possible that I got a defective cylinder, if it could possibly be the master cylinder or if I just need to bleed the system again. There are no problems getting it in gear either, and the pedal is very high, higher than normal. Any advice would be greatly appreciated.