Just want to pass along that I am seeing this problem on a few computers (about 5) as well. This seemed to start with 3.5.1. On these particular computers, uninstalling/reinstalling does not help. Over time the CPU utilization gets higher and higher to the point the computer is bogged down and causes stability issues with other apps. Removing MB seems to be the only solution to get the computer to run normally.
On one of the problematic computers I did notice that MB had lost its registration info. Upon reinstall and re-registering, the program seemed normal again.