Strange vibration still

[tomc at satx.rr.com (Tom C)]

Hi all,

"The Project" has had this strange vibration since I got it I think. RPM's
above 4.2k it hit's a "brick wall" and has a major engine vibration. This is
sudden and even more pronounced with all new motor mounts best way to
describe it would be just like a rev limiter. If I back off the gas to under
4.2k it goes away just as suddenly and drives fine. I have GX engine, with
larger throttle body, I ported the intake manifold to match the throttle
body, I have the stock points distributor, and stock 1.6l airbox and fuel
distributor.

Anyone have any ideas?

TIA,
Tom C
'79 "The Project" Scirocco