humppa,
This behaviour is definitely not normal, it looks like a bad mui4 installation to me. CVS versions are most of the time stable, and such obvious crash bugs don't stay long on cvs. Ultimately, if mui4 is well installed, you may try to delete ambient prefs (sys:prefs/ambient/ambient.prefs[.bak]]), in case you ran into some prefs conflict.
And a non-debug version won't change anything about your stability issues.