Sounding like imbalance of two or more tyre/wheel assemblies, Kev.
You've FOUR of these beasties rotating at high speed and it may well be that you've two or more unbalanced assemblies; when two of these out-of-balance assemblies work together they may well equal themselves out i.e., "sometimes gone" and/or work against each other i.e., "really bad".
And the fact that you can "drive through it" is indicative of tyre/wheel imbalence.
A classic example recently with 4-new tyres on an RS3 with slight vibration at the front, so we changed 'em front to rear just to evaluate/prove, and the slight vibration from the front went to heavy vibration on the rear that could still be felt at the front. So that car went back to the tyre fitter/wheel balancer and ALL four assemblies were balanced again, and again after that (3-times in all!) before an acceptable balance was achieved.
I'd get ALL four checked and/or re-balanced by someone who knows what he/she's doing.
And, if it were brakes, steering, or suspension, then I doubt you'd get the variations you report...
