Since I've upgraded my MacBook Pro from Snow Leopard (10.6.8) to Mountain Lion Apple Mail has been behaving strangely – Activity Monitor shows Mail consuming about 100% CPU for hours at a time. Mail is often very sluggish at displaying messages. I've rebuilt my mailboxes (Mailbox > Rebuild) but that doesn't seem to help.
Is anybody else experiencing this? Any ideas about how to troubleshoot/fix it?
MacBook Pro (15-inch 2.4/2.2 GHz), OS X Mountain Lion, 4 GB