Also check your graphic card for dust.
Artifacts usually come from either overheating gfx card (my old old 6600gt used to run 105c in oblivion and started to give weird results, i swapped the cooler to a zalman and it dropped to 60 - removing artifacts) or faulty components.

I'd start making sure the gfx card can handle the load, also lower the backgroundfps on your slave clients to as low as you can and still have acceptable updates, perhaps even as low as 10 can be playable