One lagging instance

I'm running one copy of WoW on my laptop and four on my PC. PC is a Pentium single-core 2.6Ghz with 3Gb of RAM and an GeForce FX5900. I use Keyclone with maximiser to put my main's window on the left and the three other toons on the right.

The problem I'm experiencing is the 4th instance on my PC (always the 4th for some reason) sometimes lagging terribly. It'll be working fine for ages and then suddenly be getting 0.2fps, constantly losing follow as it can't keep track of where the main is. Obviously my hardware is not optimal but I've got my team to lvl31 so far and really want to get all 5 to 60 for level-granting goodness. After that I'm happy to cut back to 3/4-boxing and eventually I'll only be keeping 1-2 accounts, so upgrading hardware is something I'd rather avoid.

I keep graphical settings pretty low (minimum except on main), and use as few addons as possible, close down everything else on the PC that I can. The problem occurs more frequently in town but also happens sometimes while out questing. I'm just wondering what more I can do to track this down and work around it until I've hit 60 with my team.

Do you have:
multiple wow directories for your toons, in the most ideal case also on different HDs? Do you have set the max fps for all windows? Esp. the max fps w/o window focus?

Seperate directories, but all across my single RAID. Maxfps and maxfpsbk are set quite conservatively I believe but I could lower them further if it was likely to help.

It's strange, if only one client is dropping, I'd guess it means it doesn't get access to a certain resource (like mem or hd). OTH, if it's always the same toon that starts to lag, that's unlikely.
Is anything else in the area of the window used by that toon? Try changing the position of the window to see if it's position related - maybe some other application is trying to do something in that part of the screen (ok, wild guess here).
Have you checked your performance meter to check if any resource was hit hard at that moment?

But honestly, if it's always the same window that starts to bug, I'd check if is toon or position related. If it's toon related clean up the cache and interface folder for that toon. If it's position related.. well, then you need to check what other program is causing the trouble (like the update message of a virus checker popping up from the tray, a RSS reader rollout, something like that).

I occasionally have this problem too.

Try typing /console reloadui on your lagging instance; this often works for me.

Probably something to do with having a single core processor and it can't multitask well enough.

It has nothing to do with being a single core.

Are yeh running anything else?

have yeh setup /console maxfps 20 /console maxfpsbk 5 on all of em?

Whats the res yer using on all of em?