MySQL Can't create a new thread (errno 12) – Plesk

I’m runnning MySQL 5.0.45/ PHP Version 5.2.6 / CentOS with Plesk.

I get occasionaly a burst of “Can’t create a new thread (errno 12); if you are not out of available memory, you can consult the manual for a possible OS-dependent bug”

Global memory is not the issue since I have about 620M free.

Anyone know what could be the cause for this?

Thanks