Strange Problem Ubuntu16.04/RX480 custom rom

I have the Problem that after ~5 hours the hashrate halfs, and short after that it goes down to ~0.9mh/s and than the system hangs.
2 Systems one with 2 cards one with 6 cards, differnt roms always the same Problem.

System: RX480 ref
Mobo: Asroch H81/97
Claymore v6.4 eth only

GPU_FORCE_64BIT_PTR 1
GPU_MAX_HEAP_SIZE 100
GPU_USE_SYNC_OBJECTS 1
GPU_MAX_ALLOC_PERCENT 100
GPU_SINGLE_ALLOC_PERCENT 100

conf:
-epsw x
-ethi 8
-eworker Home
#-dpool
#-dwal
#-dcoin sc
-esm 1
-etha 2
#-dcri 20