[L-Spec]

When my clutch was slipping, I had the same symptoms, except it would grab and throw me back at all. It just felt gutless. To make sure it was slipping (and give myself a for sure reason to get a racing clutch), I launched in fifth gear and by the smell of it, I certainly had a slipping clutch. Make sure you replace the throw out bearing while you're in there, it'll save you stress in the long run.

Not recomemnded, but if you want to make sure, cruise at a low RPM and then give it gas, it should steadily rise until boost kicks in. If it rises faster than usual, then it's pretty much a tell-tale sign of slippage. I'm drunk.