The processor may be an issue, but you'll find that 4 boxing with 4gb is fine, 5 boxing with 4gb can create exactly the issue you are describing - particularly in crowded areas.

I was able to get away with 5 boxing a single system into TBC but anywhere crowded like Org or Shattrah would play hell on it. Once Lich King released it's not feasible to even 5 box remote areas on 4gb of ram... especially if you are on a 32 bit OS.