From someone who is not a computer expert, just a general user, perhaps my experience will help.
I had similar problems, usually with my favourite sites (pprune and my home page) and I had to reset my router. For me that was turning it off at a reset switch, waiting 10 seconds then turning it on. The sites would work again then for awhile.
Eventually we replaced the router.
I guess this is a bandaid offering, rather than a real answer.
Good luck.