i have dsl service and am pretty computer savvy. here goes. its a weird one.
i have two hard wired computers to a c1000a centurly link modem/router"wireless also" combo unit, also we use one wireless laptop, and have 3 phones that tap into the wireless along with one tablet that uses the wireless also.
problem is, one computer that is hardwired when the cat5 wire is plugged in, will make the internet extremely slow for the whole network and sometimes even completely crashing it, forcing me to unplug the router/modem to reset it to get the internet back on, this computer is one i built just this last christmas for our daughter and the problems started some time after that was hooked up, not sure how long after.
i used to install some network wire and put on the terminations and such so i just happen to have a cat5 cable tester that tests all eight wires from end to end, by hooking a receiver to one end of the wires and the main tester to the other end, it tests good. also i have checked that wire physically from one end to the other for any physical damage and there is none, this wire goes straight from the router to the pc, there is no wall jack, just a hole drilled under the carpet at the edge of room, so just one straight wire, maybe 50 foot max. it was a new wire i made from a spool.
funny thing is, even if the computer is shut all the way down, if that cat5 is hooked up, we have these problems, if you unplug the cat5 from the pc, problems are gone, completely. i have tried the laptop on that wire, turning its wireless off, and that works fine too and doesnt cause any network issues, so the wire is fine along with the port on the router..its the computer, be it hardware or software, that is the question. i has windows 7 64bit on it. pretty fresh install, no malware or anything almost positive, we monitor her usage on it quite closely.
any ideas what it could be, my guesses would be either bad network port, which is part of the motherboard, its not a separate card, or something really messed up with a network setting in it, although it's all stock, didnt set up anything special with it for network access, just simply installed windows and hooked up to the ethernet port.
my ideas on how to troubleshoot this, since the wire works good with other computers and i am certain it is good, is to try a standalone network card or get a wireless adapter for it and see if the problem goes away. i cant think of any network settings to check that would cause this sort of problem...
Please help with any ideas, i have never had one quite like this, especially since the computer can be shut all the way down, and still cause the problems.