Update:

1. radeon.dpm=1, compton backed: both "xrender" and "glx"

Crash occurs.

2. radeon.dpm=0, "glx" compton backend (did not feel the need to test xrender)

I have been working with a VNC session over few hours  quite heavily (which
usually causes the crash very quickly) and crash has not occurred. 

So it looks like it is DPM-related, and my guess is it occurs when the
frequency stepping is changed.

However, with radeon.dpm=0 the GPU temp is about 15-20 C higher, even with
dynpm on.