I think the cause here is probably a known problem. The vacuums in
parallel with the long-running transactions would result in periodic
sinval message queue overflows, with resultant flushes of syscache
entries in all active backends. We know that there are places where
syscache entry pointers are used longer than is safe --- ie, it's
possible for an entry to get flushed while some routine still has
a pointer to it. Finding all these places, or better redesigning the
syscache mechanism to eliminate the issue completely, has been on the
todo list for awhile.
In the short term I'd recommend that you avoid vacuuming system tables
while there are other open transactions; that should reduce the
incidence of overflows to a livable level.
regards, tom lane