what kind of computer do you use? I have photoshop cs6 and Ai cs6 open at the same time webx5, my thunderbird + 20 tab of google chrome and at least 10 folders are open at the same time, my cpu runs under 10% !

are you using a laptop?

I have had this problem as well. Select the project with the problem, then try holding the 'ctrl' key down and press the 'Preview' button. This seemed to clear the problem for me. It seems that this refreshes the files that are generated by X5.

X

X5 is running on an 8 core workstation under windows 7x64.  Through task manager I can Identify the X5 application taking 97% of CPU, while CPU is pegged at 100%.  Further moonitoring proves that this behavior can start at any time while X5 is running.  It usually starts sucking the CPU after about 5 minutes of execution.

I have 4 core 2,8ghz with 12 gb of ram, no problem what so ever!

try to clean up your hard drive, maybe x5 files are all over the place in your hard drive (when you were installing it). what was the last time you did disk difragment?