Is my misery due to X.org, KDE, or Kubuntu?

I am a long-time Linux fan and have been a happy Kubuntu user (I prefer KDE's configurability to Gnome's) since Breezy Badger. After upgrading to Kubuntu 8.04 (Hardy Heron), however, my system has exhibited an exasperating tendency to require a reboot about once per week. After several days of use, the GUI becomes rather sluggish, and the window decorations start dissolving into garbage images for seconds at a time before being restored to their proper forms (i.e. full-screen, iconify, etc.). Sometimes I can get it to stop by closing Firefox, kmail, pidgin, etc. and restarting them. But it always ends in tears. Eventually the GUI grinds to a halt and the X server crashes so badly that the machine requires a reboot.

I have taken to logging out and logging back in (thus restarting X) at the first sign of this behavior, which is excruciating due to my use of 4 displays and 9 virtual desktops, typically covered with dozens of windows for the different projects I'm doing. Even with KDE's Save/Restore Session feature, it's painful to set everything up again.

I've been struggling to fix this since it started, but have had little success. Regular work seems to get in the way. I will readily admit to some embarrassment about "living" like this. I'm supposed to be a crack computer engineer!

It's hard to remember all the things I've tried, but here are some:

• upgrading to Kubuntu 8.10 and 9.04 (fail, but that's another story)
• tracking memory usage in the X server (I might have found a memory leak but the problem happens even when X memory usage doesn't grow)
• fooling with xorg.conf options for my 3 different video cards

I learned a lot, but didn't solve it.

Things I haven't tried but should:

• try Gnome, KDE in another dist, xfce, or my nostalgic favorite, fvwm
• disable one video card at a time to try to localize the problem to a particular driver
• enable debugging output for various components of the display system