Balancing isn't usually linear, something to do with harmonic balance I think, theres usually a point where its worst, say 70mph, then 60 and 80 might be much better.
Your issue sounds like a driveshaft to me though, you'd know if you hit anything hard enough to knock the balance/tracking out. Edit - Unless you've lost a wheel weight of course...
Your issue sounds like a driveshaft to me though, you'd know if you hit anything hard enough to knock the balance/tracking out. Edit - Unless you've lost a wheel weight of course...