Hi VGA,
Don't worry about the late response. I seem to have fixed the problem but probably not in the best possible way. It appears to be a timing problem, so changing the graph to update in real time (to slow things a little) and also adding a 15ms delay seems to have prevented the crashing. Unfortunately the time delay has made my instrument a little slower and less accurate so it's probably not the best solution. Keep in mind that it's running in open loop mode - not sure if I'd have the same error in closed loop mode.
