I'm having exactly the same problem. I've tried different AMIs and sizes, different roles, re-vulcanizing etc. The problem occurs 90% of the time on almost any fog call (assign static etc.). Once in a great while things work. No rhyme or reason...
I thought it was because I was running in a VM and I had a time sync problem. Nope. I tried things on a regular machine, and checked the clock. No issue.
I've got a machine that is down at the moment, and will probably roll back to get it up again.
Help!
Nic