It seems like I have two independent problems which may explain the confusing
results I got:

* with DPM enabled I get hard locks on suspend and sometimes on resume. After
suspend I have to turn off power manually but it seems like it successfully
writes the suspend image to disk. On resume it sometimes locks with disabled
output, sometimes it works.

* Regardless of DPM enabled or not I get X server crashes within minutes after
a suspend/resume cycle. This happens with kernel 3.11.1 with current firmware.
When I downgrade my kernel-firmware package to 20130114git-1.2.1, this problem
vanishes. But the firmware may be the original cause. With the old firmware I
for example do not have direct rendering or acceleration.

At least I found a logfile giving more information about the X server crash.
Attaching.

Is there anything else I can do to debug these problems?