Vibration at about 75-80mph

[KingNine]

Odd one this, my 90 8V'er occasionally vibrates/shudders horribly at about 75-80mph indicated, backing off momentarily OR shoving the throttle down gets rid of it, if you then try to provoke it by speeding up/slowing down, it doesn't repeat!!!! WTF? I've spent my trip to work trying to get it to happen when I want so to get a feel of what's causing it but some days (like the last couple) it hasn't recurred.

I know the suspension is in good order (replaced much of the relevent bits in the last three odd years) & passed MoT last month no bother. I serviced it before then and found nothing much amiss. Doesn't feel like a wheel balance type issue as it only happens now & then (tyres & wheels all OK, TP's fine etc), TBH it feels more like a fuelling problem but again, I would have thought that would happen all the time and would be rev rather than speed related.

Any ideas?

[tisme]

Could it be the coil breaking down? I had a similar thing just after I got mine a few years ago, I replaced the ht leads the one that goes to the coil fell a part in my hands. Just a thought, 

[adams mk2]

i think it might be the wheels check to see if they have the wieghts on them if not they wil need wieghin again

[Thom89]

Knackered shock absorber? wheel bouncing up and down..
Thom

[Gti-Grimes]

Wheel balance, just had same problems

[KingNine]

Thanks for the above. I'm sure it isn't suspension related asthe usual suspects are very low milage. I haven't checked for a missing wheel bal weight but will do so. I'd be surprised if it were that tho' as I would have thought it would happen all the time at the given speed but it doesn't, only rarely. Also, it 'Feels' like it's engine related rather than chassis related.

Any further suggestions would be welcome.
Cheers

[bodga]

Knackered shock absorber? wheel bouncing up and down..
Thom

if its any help mine does that vibration stuff at speed, and i know my back shocks are knackered.
Its on my list of jobs to do

[fused]

This sounds very similar to what I had, mine 'seemed' to be an up/down vibration rather than a front to back hesitation/hic up. My near side shock (fk coilies) had a slightly damp top but other wise every thing was checked ok. Never did find the fault but it went away in the end or did I just stop driving it at that speed  ?? I believe it was a resonant frequency (certain speed that the suspension couldn't control) of that slightly poorly shock that caused it but no way to prove it!

best drive faster then