Now you mention Outlook interaction, I'd look at that carefully. How well does your exchange server perform? I've had problems with Exchange becoming temporarily unavailable. Might be worth inserting some timer statements and logging on those procedures to check that they are working as expected and executing in reasonable times.
ODBC link thru which you can create folders? That sounds highly unusual to me but that could be me showing my ignorance.
On tabbed forms with subforms there are some tricks to speed them up and reduce memory usage.
e.g. only setting the sourceobject for the subform when the tab becomes active and resetting it to nothing when the next tab is activated.
With such a complex project I'm not be surprised that the errors are hard to replicate, especially if the form in question loads many recordsets and the data is dynamic.
You could kludge your way out of it by installing more RAM in the TS. That could be cheaper than all the hours of developer time needed to debug this. I know it sounds like a crap answer, but it may be that your TS isn't really high enough spec to support 18 demanding users (and networked database users are usually demanding). That would let your users continue for longer, to see if the memory peaks and stabilises or just keeps increasing.
