Those shouldn't be causing the problem after 5min then. Couple of things to try:
1.) There is one thing in Super Cache locked to 5min, it's the "Check front page every 5 minutes" on the plugin debug tab (wp-admin/options-general.php?page=wpsupercache&tab=debug). Do you have that enabled?
If it is enabled can you disable it, then see if the problem still happens?
2.) While you're on the Debug tab, you may as well enable the Debugging log (** for your own IP only) and then post those messages here if the problem is still happening.
3.) You only mentioned memcached, but do you also have APC, eAccelerator or similar opcode caches installed?