Just a guess, could ( some of ) the connection info be going out of scope at some point?

Also unlikely :)

Although, cranking up DBI trace level and waiting for this to happen again might reveal more

I think its a socket issue , the os manages sockets, they could get stale/deadlocked/whatever, and then the DBD erroneously reports access denied (maybe its a socket creation access denied)...

More diagnostics to really figure out whats going on

scheduled reboot (cron?) to void the issue :)