First and foremost, I hope you are not testing your clutch for slipping by pushing the pedal down hard at a gearing disadvantage (ie... trying to floor it in 4th or 5th below 2500 rpm). It will always slip, but what's worse is that you're putting incredibly bad loads on your motor, not only are you putting physical stress on all components from engine internals (rods, crank), your clutch (which is why it slips) and your transmission... but on top of that you are putting your engine in a place where it will likely knock under these conditions and knock under loads like that can very easily break a motor. You should always downshift to accelerate and generally use higher RPM ranges when driving aggressively (above 3500 or so).
Second, about as important: think of your clutch as a fuse on an electrical circuit for your transmission. When you mess up driving or shift abrubtly or launch hard, the clutch slips a little bit and then engages. Now think about putting a stiffer clutch (even at stock or near stock power levels) and that power delivery becomes instant which feels nice, but this is exactly what puts the transmission under a high risk of failure.
The stock clutch holds a lot of power if you drive it correctly. I currently run 300whp and I actually downgraded form a Stage 1 clutch to OEM equivalent and it only slips when I launch hard with Anti-Lag engaged and/or trying to floor it below 3k in 4th/5th.
My vote goes for stuck clutch!