I've added bug#24 to the known bugs list: It's an issue I found several months ago and have already reported to MS, but I've only just got around to finding the problem, the workaround and creating a Repro for MS. It's basically another timing issue, which causes Sidebar to get into a locked state taking 100% CPU:
Bug# 24: Gadget may never appear on the sidebar if you have overlayed RGB transparent images and are updating the screen too often.
Bug#24 - Gadget may never appear on the sidebar
Hokgiarto