turning off the graphics accelerator has helped, so far no further crashes! Not usre, what GFX warnings you refer to, I did not see any warnings, so I could not have ignored them....also, I have no other graphic app runing on this MAC.
One sidenote: I had on the original problem (dock eats memory and CPU) one more occurence, until I rebooted all machines at the same time after adding the line suggested by Steve Goddard. I never experienced the memory problem, but as mentioned earlier, I have 16GB RAM.