I have two computers in my room. I have a 10/100 switch is in the room next to mine. There are CAT5 cables running from my room to the switch. The cables are about 10ft in length. not much. Here's the problem. When the auto-negotiation takes place, the switch senses a 100mbps Full-duplex connection, which is what I want. But, on the switch, all three lights(Link ACT, 100Mbps, & FDX/COL) blink. It's like the connection is good for about 2 seconds, then it turns off for 2 more seconds. This repeats. I've tried different computers and NICs. It's all the same. When I force the NIC to do 10Mbps(full or half duplex), the connection stays solid. But I don't want a 10Base connection, I need 100. The funny thing is this: When I bring the switch into my room and use different cables, 100BaseTX-Full-Duplex works great. The point is, only the computers in my room are the ones that get messed up connections. The other comps throughout the house work fine.

Are the cables running next to power lines in the wall, near speakers or other equipment that might be interfering. Are the cables actually built in wiring with jacks? They might be the wrong type of cable inside the wall, or have their shields shorted together or to something inside the wall (or even making longer runs than you suspect). The plugs could be loose, dirty or otherwise damaged. What happens when you only connect up one of them at a time at both ends?

Jack is right: the cables are looking good for this.

Hi, I had the same problem at a customer location. network works fine for some time, then whoooshhhhhh.I did the same task of adjusting all NIC 2 10mbps and it worked fine for over a month..Gladly! Then back 2 square one. Finally i replaced the damn switch. It seems some induction coil is the fault. Well ur prob seems strange. I strongly recommed u check out the earthing level. If possible, use the same source of power for all the devices.i.e use an extension cord to connect the switch in the other room drawn form same source for PCs.