I first noticed this problem when I was driving to Ithaca this weekend, and I am able to reproduce it nearly every time under the same conditions.
I'm in 5th gear at 2500 RPMs. I press the gas 3/4+ of the way in. RPMs and speeodo climb normally. At 3000 RPMs they spike to around 3500 and then fall back down. The speedo doesn't jump with the RPM spike and everything continues normally once the RPMs drop back down. This doesn't happen if I started at 2250 RPMs or 2750 RPMs or in any other gears.
From searching I think that this indicates a clutch that's beginning to slip and is on it's way out. Does this sound like the right diagnosis?
I have a 2002 bugeye wagon with 51,000 miles (about 8,000 of which are mine), Stromung catback and cat-less midpipe.