Not to hijack this thread, but I have a similar problem: Last night, I stripped out the front wheel of my partner's bike, and regreased and cleaned the bearings. I only took one side of the axle apart, so I wouldn't have to mess with centering issues, and put it back together again. It spins fine in my hands. Put it on the bike, and all of a sudden there's a kind of bouncing thing going on. The front of the bike actually moves up and down, several times per rotation of the wheel. It appears to be in the dropouts correctly, and I had the damn thing apart so frequently it's driving me MENTAL.
I'd had to deflate the tyre (big ol' MTB knobbly ones) and re-inflate in order to get it beyond the brake blocks, and it does seem that reducing the pressure makes it happen less, but I'm not convinced that that's what the problem is. Anyone got any bright ideas?
I'd be most grateful (and out of the doghouse!) if you could shed light on where I've messed up.