I could not find a way to force garbage  collection, but there is a
similar problem discussed here:
discuss.fogcreek.com/.../?cmd=show&ixPost=72832
They mentioned using DoEvents and looking at memory  usage in the
task monitor. There are also MemoryTotal, MemoryFree, and MemoryUsed
properties you can look at.

Off hand, your app does not seem like enough load to crash VBA. Are
you using external functions? One of them may be the culprit. Or you
may have created something recursive by mistake.