It seems to me that "Clock advanced by" is notoriously hard to trouble-shoot.
Have you run your config through iHealth? iHealth checks and provides pointers to solution articles if it finds a problem.
You might want to upgrade to take advantage of all the bug fixes.
We had lots of problems with 11.2.0 and upgraded to v11.3 right after it came out just before last Christmas.
V11.3 adopted a threaded model, which is different kettle of fish. There are not a lot of hardware threads on the 6900, so v11.3 might not help you much, capacity-wise. I'd go for v11.2.1 HF5.
-Jie