Are you sure it's the router? Is a wired connexion also suffering? It is wireless that's the problem, there's quite a discussion on wireless here:
http://www.cableforum.co.uk/board/12...wn-page-3.html
Not the same condition as yours (which might yet turn out to be peak time congestion on VM - but then only if wired is as bad), but it's informative.
The acid test as to whether or not it's the router would be a direct connexion to the modem at the time of day you experience the problem.