Not sure I can help you but I can tell you that I too had problems with the latest update to Leopard (10.5.5). It broke one of my applications (hurricane tracking program) and has injected instability into certain power settings that I use with an external monitor.

Most folks have installed this update without problems but there are quite a few reports such as yours and mine in other Mac forums. You might want to read through some of them on the Apple sponsored forum.

I had made a complete backup before installing the update so it was easily removed without difficulty. I'm back to running 10.5.4. I think I'll wait until Apple publishes 10.5.6 before I attempt to update the OS again.

Regards.