Here's an excellent post on that, including instructions on how to diagnose it:

http://blogs.msdn.com/ntdebugging/archive/2006/12/18/Understanding-Pool-Consumpt
ion-and-Event-ID_3A00_--2020-or-2019.aspx

The most likely suspect is a handle leak. The stable way of solving that is
to find the leak and make sure all objects are properly released, but if the
leak is in the VB6 code you may be out of luck, and a periodic restart would
be the only way of "fixing" things.

In the .NET code, make sure you properly .Dispose() objects that implement
IDisposable, and use safe handles for unmanaged resources (see
http://msdn2.microsoft.com/en-us/library/fh21e17c.aspx). If the issue is
that COM objects are being kept alive when they shouldn't, check if you're
keeping references to them that prevent garbage collection. You can also try
explicitly releasing the objects through Marshal.ReleaseComObject(),
although this should be necessary only if objects need to be freed in a
specific order.