by Mark Pearson.
David,
I did a Google search "php error Cannot obtain session lock" and found several links to items from the Moodle Tracker which may provide some useful info, eg
MDL-30026 Prevent session lock waits from stalling server processes forever
When you find a cause for this behaviour (and hopefully a solution) please post it here -- I'm sure others will be affected by the same problem.
Mark
