Hey guys I was wondering if you might be able to help me.

Today I started running a second account on my Vista64 machine. It has a quad core Q6600 and one of the new Radeon HD4850s along with 8gig of RAM. I run a dual monitor setup.

When I run two instances of WoW at a time, it will work fine for a while. The problem arises if I alt-tab to Firefox or something -- and even then it is somewhat random and doesn't always occur. But when it does happen, the entire computer becomes choppy instantly. It goes from both instances of WoW getting 60+fps and perfectly smooth to the whole computer moving at 5 fps (even firefox, etc.). Sometimes I can wait it out, but other times I must close one of the WoW instances for it to return to normal.

One thing I notice when the computer is lagging is that instead of each of the WoW instances taking up 15-20% of my CPU power, they take only 0-3%. When the lag stops, they return to 15-20%. I'm thinking that when I minimize/alt-tab away from one of the WoW instances, it's CPU priority gets reduced or something. Since the game isn't getting enough CPU power, the entire computer lags. This is just a theory, so I've tried setting process priority and CPU affinity -- no combination of settings has worked for me.

Anyone have any ideas on how to fix this problem?