I would check your ISP modem and connections, make sure the signal going into the modem is at proper levels. Get the ISP to check this. If your on Cable, make sure there aren't any TV line splitters between the ISB cable box out side and the modem. These can lower signal going into the modem. If you need a splitter, use 1 good quality one. Might check to see if your modem needs to be updated or upgraded. I prefer using my own modems however not sure if your ISP allows that. Ask. If you can, I recommend Motorola SB 6 series cable modems.
If your on DSL, make sure the phone lines are in good condition and filtered correctly.
Use CAT 6 cables between the ISP MODEM, router and wired PCs.