How Do You Deal w/ "Heisenbugs"?

horos1 asks: "I was wondering how people out there deal with 'Heisenbugs': bugs that have no logical, programmatic cause (mostly from C and C++ programs and especially threaded C/C++ programs), that may change or disappear if you modify the state of the program. For example: we have a multi-threaded C++ app which cores in about 5 places, at a different memory address each time, and which disappear if we turn threading off. They seem to be caused because of a memory overrun error, but this too is exceedingly hard to fix with tools like purify, because they tend to give several 'false positives' on memory errors, as well as core when we link with certain libraries. Anyways, this is getting very annoying... Any help with this, as well as pointers on how to deal with bugs like this would be greatly appreciated."