I've got the shakes!

[entropysix]

So to get things started, I recently (this summer) purchased a 1988 Supra Turbo (117k miles), and so far it's been pretty sweet. Just about everything is still stock (although looking over the receipts I got from the previous owner, it seems that the radiator was replaced a few years ago).

However, when I start getting close to 3k RPMs (it's a manual, by the way, the problem seems to be at it's worst in 5th gear), I can feel the car start to shake. Once I get past the 3k mark, the shaking stops completely. It is something I noticed first in 5th gear, and it is progressively less noticeable in lower gears (can feel it in 4th, a little bit in 3rd).

So what do you guys think could be causing it?

~Kc

edit: Also - it seems to only be doing it when I've got my foot on the accelerator (and it'll stop immediately if I take my foot off).