Your CPU/RAM/Power Supply could be overheating due to the open ventilation.

Computer cases are designed to stream the various air intake fans across important device's heatsinks to assist whatever heatsink fans exist.

With an open case, you are lacking the front intake fan(s), the side intake fan, and any additional rear ventilation fans, found in most cases.

Without the air streams of a case, your ram is not getting any ventilation. The gap between the sticks of ram is almost enough to cause an area of motionless air, causing the Ram to actually be insulated and retain heat.

Its probably not as bad as I'm making it sound, but heat might be an issue.

Although, you did say that it stops once you restart WoW, so maybe an Addon you are using is causing a memory leak?