I just played 50 twice, each time I had turned the corner and had my castle up to level 6 and was on my way to winning and boom, lockup. Anyone had this problem or have any ideas? Have had zero probs up to now, been completely stable otherwise.

I just experienced a similar issue, I fear. I only started playing the level in earnest a short while ago, so I've only seen it happen once, but if it's a recurring issue then I'd be very interested in a fix for it...

[edit]
I just spotted a potential solution in post four (the last at time of writing) of this thread, which essentially amounts to "turn all settings down and lower the cycles".