found a bug description which describes my problem exactly:
Frequent problems with dbus-daemon entering a runaway state, consuming 100% of
one core for extended durations. KDE applications fail to open or react in a
timely manner while this is happening. Screensaver never happens.
Steps to Reproduce:
Haven't been able to identify a particular usage pattern. Typically takes several
days to a week to reoccur. Always happens eventually if I stay logged-in for significant periods of time.
Seems linked to login duration. Running KDE apps (or dbus enabled apps)
include: the KDE desktop, Krusader, Konsole. Common non-KDE apps include: Firefox, Chrome, Eclipse, VirtualBox(non-OSE), Pidgin, Skype, Dropbox, System Monitor.
One core at sustained 100% cpu. Launcher won't start new applications, usually have to reboot from command line.
Restarting the daemon doesn't seem to have an effect.
While killing it makes the system unusable irrespective of a future daemon restart.
dbus shouldn't hog CPU
restart dbus should return the KDE env to a viable state after killing the process
KDE 4.5.1 on Ubuntu
8 GB or RAM (probably unusual)
In the absence of a way to reproduce, marking as duplicate of the most likely cause.
*** This bug has been marked as a duplicate of bug 23194 ***