Maybe try using a binary search debugging strategy?

From http://stackoverflow.com/questions/843909/debugging-and-binary-search

If you don't know which line in a 100 line program is buggy, you'd try to run the first 50 lines and skip the rest. If the problem show up, you'd know this first segment contains the bug. You'd next try to split this and run the first 25 lines and see if the problem is there and so on until you have figured out a short enough piece to look at