No *real* stability issues for me:
I have a "wubi"/Ubuntu 9.04 64bit installation and have had two stability issues: a screensaver lockup (likely sue to my ATI X800), and I had to delete my second ethernet because it would lose connection (probably confused as which one to use).
I have a ubuntu 8.10 to 9.04 upgrade on a virtualbox VM, no stability issues but hardly used.
And the one i use most is still xubuntu 8.10 64bit with forced upgraded xfce on parrallels for mac VM at work -- it mostly had no issues except when the osx 10.5.7 update and paralles update both came out... had to uninstall parallels and reinstall, no stability issues since.
:dunno:
good luck with solving your issues.