Hi,
I've been experiencing the same thing on my iMac but it's gotten much, much more noticeable lately. It might not be worse, but since I added iTunes Match it's been a real pain. Basically, I'm trying to replace all my lower quality songs (about 3,000) with iTunes 256kbs versions. When I'm lucky, I can get iTunes to download about 100 or so songs but then I get a spinning beach ball which hangs everything. I end up having to do a hard reboot each time which is making this very frustrating.
I'll keep my eye on this thread to see if anyone finds a solution/workaround.
BTW, I'm running the latest version of iTunes (as of 18 Nov 11) and Lion is fully up-to-date as well. If it helps, it's a 24-inch early 2008 iMac with 4 GB of RAM.
Best regards,
Anthony