A few things to check:
Have you verified that vncserver is what is using 100% CPU? Most likely is the case if you don't see it when you auto run, but worth a shot to check. Try running this command and paste the output here.
Another thing is to check the log files in /var/log and see if there are any details on vncserver. I can't remember if there is a specific vncserver log but if there isn't syslog.log and user.log might be useful.