Lovely little problem I'm developing.

So for some merges/passing (I don't drive THAT agressively), I'll take the car up to 5k RPMs and shift - it's a sports car, after all. I don't do this every gear all the time, of course - only if I have to pass or I'm merging on to a freeway.

Today, around that shift point while in second gear (at about 5K RPMs), I noticed a vibrating noise coming from the shifter/boot area, and when I grabbed the shifter to shift... it went into neutral. I had not yet even touched the clutch. I replicated the problem again - high-ish RPM's, vibrating noise, and I can dump the car into neutral with minimal/no effort and no clutch needed. The vibration also seems to occur in first gear. I should mention that the car only has 23,000 kilometers on it, and overall is in extremely good condition.

This is not a good problem to be having...

Any ideas?