Make server.php die when it fails
I've just finished reinstalling a couple of my servers and had some troubles with one of the slave dns servers that didnt have permissions to connect to the mysql and/or run any queries.
I had multipul instances of the script running that caused a huge spike in CPU and RAM usage. The process' hung and were eventually killed, when i've gone through and debugged the problem, it would seem as though if the database connection fails for whatever reason (in my case it didnt have privileges to use or run queries on the database) it repeatedly attempts to run the same command/query over and over.
My suggestion is to make the script die and show a warning in the ispconfig log if it fails instead of the crontab opening hundreds of instances that hang.