I tried to upgrade stuff, but error occurs again and again...

My websites keeps crashing periodically (I get a blank page) and the error log reports a problem with joomsef.php script.
//==========
[Thu Aug 13 20:36:47 2009] [error] [client 83.26.65.114] ALERT - canary mismatch on efree() - heap overflow detected (attacker '83.26.65.114', file '.../public_html/components/com_sef/joomsef.php', line 346)
//=========

I have to restart server to make site work again.
I have JoomSEF 3.3.0 and extension fo virtuemart.
Could you please shed some light on the issue?