Here are a few of the things you could do to troubleshoot the problem: Try switching eth cables, try switching the phone cables, try keeping your router ventilated (nice cool air as opposed to a heated area) and if all else fails have a line check done on your phone line.
One of the things many people don't realize as the last course of going through all the typical hotspots is that, when their is some form (any form) of static on a phone line, chances are you're going to get boinked off. Speaking from someone who works at an ISP, what area are you in. I wouldve posted LATA but in all honesty chances are you wouldn't understand what I mean...
Visit Nanog and check for outages in your area as well. ISP's have been known to go down if its merely a connectivity issue, so search Nanog for your ISP and if by chance you know your Lata search for it on sites like Nanog (www.nanog.org)
One of the things I've noticed throughout time with many telco's (remember they own the line) is that when a customer is using their lines, many times a repair in the area will bork things out. As long as you have a stable link light you're fine. One of the things you could also try to do is wire your mode then your router into your 'NIT' and check for issues there too. Faulty inside wiring could be to blame.
sil
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