Vibration at speed (8v GTi)

[KingNine]

Further to my original post about severe vibration at about 75mph, I'm able to provoke it now by driving at about 70-80mph in 5th, backing off the gas a bit then opening the throttle again. Going to full throttle will stop it but backing off through that speed band will cause it to re-occur quite often. The fault can be repeated to a lesser extent in 4th at similar revs but obviously lower roadspeed.
At all other times it starts, runs and drives fine (1990 - 121K miles standard 8V GTi), in fact, it goes really well!

This is getting to be a real pain in the ass now, I've tried all the usual things from service items etc. just in case but to no avail.

Any ideas?

[Neo Badness]

Have you checked all the wheel balances?

Engine mounts?

[Thom89]

Swap wheels front to rear, then try again.
Thom

[KingNine]

That's next on the agenda, swapping wheels round then having them balanced.

Also, could these symptoms indicate engine mounts on their way out? I've never experienced failed mtgs before so don't know but imagine they would feel similar but most of the time and would be visable when stationary and the engine revved.

[Neo Badness]

Yes to above, but not always.

Get the wheels swapped around then worry about engine mounts later on.

Be systematic.

PS
I assume you have access to your own test track given that national speed limit is 70mph?

[Mitching]

I get that problem too, around 80 69