Meh, another day and the damned prob reappeared.

I've done a debug log as you suggested and got totally lost! SDL never was my thing. Anyhoo, I did a quick google and I'm guessing that a WarpMouse could be the cause of it, and it does appear in the log randomly.

Anyhoo, I've attached a cut down log (damned file size limits) cos I get the feeling you'll be able to read it better than me.