Tom, your silence suggests that you may have solved your problem.
However the last time I had a similar 'insoluble' problem I solved it by a, updating the modem/router software - it seemed an improbable reason as the router/modem had worked flawlessly for the previous years and b, changing wifi channel - it would seem that all my neighbours and their friends had got wifi and for no good reason this appeared to have an effect on the connection. Neither of these actions made any sense but it solved the problem which hasn't reappeared yet.
