Mine did the same thing. Bought the car with a dead clutch (really heavy single), replaced the clutch, fine for three weeks, then started slipping... then it got worse and worse till it was practically undriveable.
Shortened the rod on the slave cylinder, and it came good again. A month or so later now, and it's slipping in 3rd and 4th if I put the boot in.
I tried to adjust that pedal-rod, but it wouldn't budge! (removed split-pin and all) My mechanic thinks there's something else wrong though, because my pedal has about 3mm free play, then it's just stiff... all the way to the floor. Takeup seems to be right near the top of the pedal-motion.
I'm stuffed for ideas, and it sounds like my clutch has been nuked in the process. =-[