linked list corrupt on efree() - heap corruption detected

So I don't post problems here often, but hoping some linux guru knows what is causing this.

I keep getting this error is my Apache error logs:

ALERT - linked list corrupt on efree() - heap corruption detected (attacker 'xxx.xxx.xxx.xxx'

It's actually my IP.
And then 1 or 2 requests later all Apache2 instances lock up and my entire site returns 500 Internal Server Errors.

Restarting Apache fixes it temporarily, but things lock up again after a few hundred more visits. Rebooting the server does the same.

The only change I made that seemed to precipitate this is I installed the curl PHP extension (apt-get install php5-curl), but have since removed which hasn't made a difference. PHP, Apache are at their latest versions.

Anyone have any ideas how to fix this once and for all?