well it would have to be uber light if anything...as they're intended for smooth If you;'re having problems with them then id say go for some proper grind pads....phatpads, coustsinks etc
right..could be loose spokes, broken axle, flared dropouts letting wheel move, crack in the frame letting frame flex, er...loose wheel bolts, maybe nadged wheel bearings thats about it i think
to be honest i love halfords, the one in plymouth is the shit....the guy in there is awesome, gave me a free qr axle AND helped me fit it....i turn up there on a weekly basis asking for tools as well