Setuid unavailable. sshd hanging on exit.

[Jason]

I have a home server running a yum'd up to date FC6 x86_64, it has a 
problem I haven't been able to fix myself.
Sometime after about 24hrs uptime it will stop accepting mail and start 
putting messages into maillog like this -
Jan 26 10:38:51 server postfix/spawn[1281]: fatal: setuid(99): Resource 
temporarily unavailable

And when I SSH into it on exit the connection hangs and doesn't close.

I've been googling and not finding much to help.
One similar problem suggested turning down the process limit for smtp on 
postfix but it hasn't helped.

Rebooting fixes it until the next time.

It's core2duo based and so fairly new tech so I have tried running newer 
kernels (currently 2.6.20-rc7), no difference.

It's possible I upgraded from my previous servers FC5 32bit install 
instead of a totally clean install...

If anyone has any idea what's happening or suggestions to help me find 
what the real problem is it would be greatly appreciated. Thanks.

Jason