IMO, there are four main areas that will cause "network" disconnects or general wierdness like that:
  • NIC hardware or drivers
  • Graphics driver/hw compatibility with your NIC
  • Router/cabling problems inside your house (e.g.: the cables from your cable modem/router to your PCs)
  • Software firewalls that are not set to HIGH cpu priority

If you haven't already done it, try setting all your WoW.EXE processes to LOW priority and see if it makes a difference. This usually indicates a software resource starvation problem.

I would then try updating your NIC drivers, graphics drivers and router firmware first and see if that makes any difference. Next, I'd go through your NIC's advanced features page (via Device Mangler) and see if anything in there jumps out at you as misconfigured. After that, I'd try swapping your video card to the other X16 PCI-E slot (assuming you have one).

If none of the above work, try inserting a PCI-E/PCI NIC and disabling the on-board one (assuming your dell has an integrated NIC; most modern OEM systems do). Still no dice? Try a different brand video card (e.g.: got ATI? Try an nvidia).

If none of that works, the problem may be someone upstream of you throttling you or some other hokey configuration or switching problem, though that usually affects more than just WoW.