What is the best way to track down a memory leak that is only found on one customer\'s test/release box, and no where else?
If the user has the problem it consistently, take a stackdump and analyse in the standard way
Try a memory profiler like ANTS Profiler.
Here's an option: Give them a box where the leak isn't present.
Sometimes, it's not the code.
Edit: It's either the code, the data, or the configuration.
Or the .NET Framework, the OS, the drivers, IIS, or COM (automating Excel, for example), or so-on.
My assumption is that the memory leak is not reproducible except on the client's box (which the dev cannot be allowed to access for debugging).
dotTrace3.1
(This question is kinda funny, cause I am tracking a mem leak that isn't present on my machine ...)
It's either code, data or configuration.
Since you say the code is not faulty 100% of the time, I would blame configuration. Take a copy of the configuration (and optionally some data) and try to replicate the problem; you won't know you've found and fixed it without reproduction.
Finally, solve it with a memory profiler.
PerfMon can be helpful (http://dotnetdebug.net/2005/06/30/perfmon-your-debugging-buddy/). There are several counters that may help narrow down what resource is leaking, and at what rate, etc.