Make a profile with the contacts of Event Variable Set and set the condition for that variable to be equal to zero.
Within the task just write to a log file the date, time value of a variable and %TRUN
Set the launch priority for that task. Very high.
That will give you all the tasks running at the time the variable was changed.
If you are lucky you're offending task will be in the log..
Another approach would me to enable and check the run log for the time the variable was set to zero.