I'm having very similar problems (which I haven't yet solved) - only
one of my PCs has the problem - the other is always ok. One fo the
things I did to isolate the problem was to connect my "bad" pc directly
to the ISP (bypassing router). In my case I still had the problem, so
the router was eliminated from consideration.

But I must ask: have you solved your problem, and if so, what was it?