I just got my clutch replaced and am still having problems.
Why does the engine rev and the RPM gauge go up when I excelerate ?
I thought this was a clutch problem so I got it replaced. I took the car back and it did not slip while the mechanic test drove it... but several hours later it happened again while I was driving.
Can the clutch slip intermittently ?

It is possible, but not normal.
The clutch pedal may be sticking and not releasing the clutch hydraulically.
Or the clutch fork may be binding on the pivot. The tech that did the clutch would likely have noticed that before finishing his work.