Have you tried what top506 suggested and did it make a difference?

Otherwise, look closely at all the sprockets for wear. If the sprockets are good but the innards of the freewheel is not (no clicking=bad or damaged pawls) , it is possible to rebuild it by substituting the freewheel body.

Is this a two-notch or four-notch body? I might have the fw body you need, but you'll need to have the correct remover, two chainwhips, the motivation and patience.