A problem that cannot be reproduced is tough to solve. Said someone and quite true also.

Some general tips :

1. If it is POSSIBLE to disable all the other interrupts within every other ISR, then why not try that ? This may or may not be possible depending on your requirement.

2. Comment out non-critical modules and go one by one to find out the offending module- time consuming but better than scratching the head. But I have this technique quite often - divide and conquer.

3. Your Power supply - how clean is that ? Is it decoupled adequatley ? And see if the MCU hangs when somebody switches ON that tube light / drilling m/c or TV ?

And finally : In ALL such logic defying issues where I thought that the code was perfect, believe me - there was A bug - subtle although.

Good luck

Raghu