>> How can I debug this further?
with difficulty I imagine.
However, I'd probably start by turning on the "log to disk" functionality. That way you'd at least be able to see the last few incoming requests, and from there see if you can spot a pattern. You'd need to get a few GPF's to make this useful.
If you can spot a trend - like for example if there's a request to generate a report, or some specific form, then you can start looking at the code around that procedure. Better yet you might be able to figure out how to create the GPF "on demand".
Cheers
Bruce