FS9 freezes when I click F1 (cut throttles)

This is an intermittent problem but regular enough to be very annoying.Can anyone shed any light on why FS9(.1) freezes up, maybe 25% of my landings in PMDG 737s - doesn't seem to happen in any other a/c (and occurs at all airports) - when I press F1 (mapped to my Saitek X52) on landing. Instead of cutting throttle after reverse thrust, the screen immediately freezes up. I am still getting sounds but have to close FS9 to get out of the situation.One of those things that is going to be the devil to troubleshoot I imagine (especially as it is not predictable, but I'm posting in case this rings a bell with anyone.Thanks,Martin