I've never experienced your particular problem before but usually when there is some mysterious breakage in one of my programs, I print out some diagnostic info that will help me formulate a theory about the cause of the breakage. Either inserting print statements in the code or running the debugger should give you some clue.

Given that you haven't given us any information on what your program does (other than "processes a list") it's not likely that anyone will be able to help you more than to tell you "go debug your program!"