Mac – How to troubleshoot stuttering iTunes video playback while downloading on Mac OS X 10.7.2

Observations: Recently, not sure when it happened, with upgrade to Lion or before, but I have not been able to cleanly watch a video of any type in iTunes while something [movie, tv show, app, podcast, etc.] is downloading from the iTunes store. This is particularly annoying when renting a movie, and trying to start watching before the movie is downloaded. There have been no hardware changes, and playback used to work perfectly during download. There have been a lot of recent changes to iTunes, Mac OS, and I would like to fault isolate. How can I determine which software might be causing this problem, or if something is interfering with playback?

Problem: iTunes video playback is choppy only while downloading anything from the iTunes store. Video is choppy at all sizes, e.g. actual size, full screen.

System Setup: iMac 20-inch, Early 2008, 2.4GHz Core 2 Duo, 4 GB 800 MHz DDR2 SDRAM, ATI Radeon HD 2400 XT 128 MB, Mac OS X Lion 10.7.2; iTunes 10.5.2; iTunes library stored on external Firewire HDD, HDD approx. 50% full; All software up to date.

Solutions already tried: iTunes reinstalled; Quicktime reinstalled; reboot; All other apps closed; iDevices ejected from iTunes panel;