I took my car out for a drive today, and i experienced what felt like clutch slip in 5th gear

(only 5th gear so far) It slips between 3000 and 4000 rpm, and only when i plant my foot on the floor at around 2800-3000 revs. It doesn't do this with any other gears, and it doesn't always slip for some reason.
It happened a few times, so my question is - how can i check that it is definitely clutch slip?
are there any other possibilities it might be?
how long will the clutch last when it starts slipping?
Also - I remember seeing a thread on a clutch upgrade, but i can't remember what the thread name was, can someone refresh my memory

- Scrap that i remembered the name
Thanks, Mo