I have a Zyxel modem and Linksys router. Modem and/or router are very picky what networkcable is used. Out of 5 cables, only the one which is 30ft long works OK. All other cables are much shorter and have a downloadspeed that's OK, but upload is nearly zero. So I pull my hair out...
so u tested the resistance on each cable? router firmware then. i had a linksys that would do similar.. was the auto sense 10/100 speed selection. driver also for lan card.