PDOException Deadlock
I had some similiar issue.

Problem was that drupal was cosuming 100% of cpu usage, sometimes making the execution of some queries to take to long and reaching the max timeout on LOCK_TIMEOUT

If you have acccess to command line, check proccess, cpu usages, sql usage and so on

Hope it helps

There are some threads about drupal sites cosuming 100% of cpu (Kind of old but i had this problem recently), take a look

See this thread for a bit of debuging