I've recently upgraded my server from MySQL 4.1 to MySQL 5 and since than I'm having a lot of problems with it.

The biggest problem is that server stucks every once in a while and the following error appears:

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

Server load is fine, but in mytop suddenly there are more than 300 threads and then the error appears and the threads disappear... than it goes back again every 10-30 minutes...

Does anyone have any clue why is this happening and what should I do?

Thanks