Busy sites have often encountered problems with deadlocks on the group contact cache. There were no less that 3 different code contributions to mitigate this problem put up for 4.7.8 and a number of other discussions have been going on in JIRA.
Merged into 4.7.8 are some improvements which we hope will mitigate this problem for those sites that experience it. JIRA is the primary source of information on this, however I wanted to share a brief overview.