| Chris,
Not sure how old your bike is so this might not be applicable, but the tank/airbox rubber seal did change to a much thicker one circa "99 or "00. Wobbly tank syndrome seems to plague a lot of earlier bikes but replacing the seal to the later type cures the problem. Only downside is it can make the tank a right pig to fit.
In your case it may just be that the seal has compressed through age/use and could do with being replaced.
Hope that helps,
Neil. | 
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