Give this a look over and see if you are a match for one of those problems.
Do you have access to your web server's error_log file? If you do, reactivate those plugins and check the error_log file for where the problem is. The time stamp should let you narrow it down.
If you don't have access to that file, then it's lengthy and time consuming but try different combinations of turning on the plugins on at a time to see where the issue is.
If that doesn't narrow it down, then switch to the Twenty Eleven theme and repeat.