Help me catch mem corruption in DOS app

I have a 16 bit dos app written in C. I do not have the source for the whole program - there are stubs which are called, and i generally write code for the stubs.

Theres a global var which is getting corrupted very occasionally. I`ve pored over the source code i do have, and can see nothing which is corrupting it. I`d like to have some sort of run-time watch on the location (its actually a file handle,and is 4 bytes long), so i can see which instruction is causing the corruption - even if i just get an address of some code to which i do not have the source. Although i`m competent at C programming, i`m not an expert at DOS programming, and this code uses linked libraries, overlays etc, so i cant just use something like CodeView (i believe).

To make things more challenging, i`ve never got the corruption on my development machine - only out in the field. So what i`d ideally like is something which watches a location which is only known at runtime, and if it gets changed (other than just the once when its set up initially) a dump of the instructions at the current Program Counter.

Any ideas/suggestions?

Thanks,
Alex.