I'm pretty sure it's the known overheating problem. Shut down until it cools and it should work fine again unless it overheats again.
I carry a couple chem instant-cold packs (first aid, crush them and they'll get cold for a while, bought a number of them at the dollar store) which I plan to use if it happens again or if the situation gets really hot, but just ventilating the bag better, packing it looser, not putting my V3 immediately next to it and keeping the bag shaded with a small 'Gackbrella' has kept it from happening since I've become aware of the problem. Both the MkI and MkII seem equally susceptible.
I'm currently out of town and have been mostly off-line, but logged in and saw this thread and just wanted to help. Should be back on-line next week.