Matt,
Important: make sure it's VFP6 SP5 - many vfp mtdll problems fixed with that SP.
Check out message#627606 from Mike Stewart of Microsoft. He regularly tests VFP mtdlls on a 4 processor machine.
There's a little about multi-processor with VFP mtdlls here:
http://www.activevfp.com/dev/dna08.htm
The MS stress tool is easy to use. you should be able to approximate what they're doing and identify problem areas...

>Hi,
>
>I've hit a problem which I'm finding very hard to debug.
>
>We used VFP 6 to create a dll which is called from a web page.
>
>This seems to work fine, however the user is now testing it under heavy stress on a multi-processor server using Microsoft's web stress tool.
>
>The server uses Windows 2000 Server SP3.
>
>The web stress tool has been set up to run 200 threads. After an hour, IIS hangs. The user is reporting this as a bug.
>
>The dll uses an odbc connection to Oracle 9i.
>
>The difficulty I have is:
>
>1. I don't have a multi-processor server to test this here, and the server this is happening on is on the other side of the world from me.
>
>2. I don't have sufficient knowledge of the web stress tool to know exactly what it is doing.
>
>3. I can't tell at the moment if this is a bug in Windows, IIS, the Oracle ODBC Driver, my DLL or the VFP runtime library.
>
>Has anyone any clues how I should proceed ? I'm advised by the client that the ODBC driver has already been patched to fix a memory leak.
>
>Has anyone got any experience with using VFP dlls on a multi-processor server ?
>
>Best regards.
>
>Matt.