what a pain... I wonder what antipodes does here 
That may be worth a try.
@TwoCables: Please try an antipodes build (you'll also need to unpack this zip to your FSO directory) to see if that changes your situation. Those builds use a new and (hopefully) better way of checking which keys were pressed which may solve this issue.
I will be able to try this in 6-8 hours from now. I think.
I was getting worried that people might be waiting for me to try this and reply with the results, seeing as how active I've been in this thread so far.
Edit: Nope, I'm too sleepy. I might be able to do it tomorrow, or maybe in a few days because my life's about to get busy for a couple of days.