Closer look at ORA-600  trace file showed memory leak with "KFNS Locked AU " chunks. ASM alert log also had ORA-600 [kfnsMasterWait01] and ORA-600 [kfgscDelete00] errors. ASM version was 184.108.40.206.1 i.e. 11.1 with PSU 1.
Search for known issues lead me to couple of very bad bugs (9217933 and 9250812), already alerted on Metalink/MOS:
Note:985150.1 - ORA-600 [KFNSMASTERWAIT01] occurs after applying PSU 220.127.116.11.1.
Note:985183.1 - New patch required after installing PSU 18.104.22.168.1 to ASM environments.
Note:9209238.8 - Bug 9209238 - 22.214.171.124.2 Patch Set Update (PSU 2).
In short, if you are on 126.96.36.199.1. you better apply 188.8.131.52.2 (11.1 PSU2).