Again, follow the FAQ (as linked in my first post to the OP). Something on either your PC or router is blocking the multicast request from the PS3.
As for your issue where PMS disappears after 10-15 minutes of inactivity, that could potentially be an actual issue. Are you power management settings set to turn off disks or go to sleep? Is there a specific error message on the PS3? Please start your own thread and provide the debug.log - viewtopic.php?f=6&t=3507&start=30#p38484