sounds to me like a flywheel issue.

i'd be willing to put money on it, as a matter of fact.

You symptoms are synonymous with a worn out clutch that's just not grabbing well anymore. And you're right, it is slipping; but it has nothing to do with the clutch itself

The break in period gives it away... with a stock OEM replacement (probably an aftermarket piece from advance auto) it only took a few days to break in the friction disk surface. Whereas with the high performance clutch, it took a couple of weeks to wear down the surface to match the flywheel's. Performance friction disks are a lot harder than OEM....

Replace your flywheel with one that's been resurfaced by a reputable shop or prefferably a brand new one from Fidanza or Clutchmasters. I'll paypal you five bucks if that doesn't solve the problem.

-jonathan