Hi Jon,
I have been having the same problem with the shut off on my Meg. I did the lube thing and it helped but it still would stiffen up after a couple of dives. Andy showed me how to split apart and disassemble the valve, I was then able to clean all the surfaces and lube it with some O2 lube. It has been working better but it still gets hung up at times. I found that by rotating the sleeve it would free up and operate more efficiently. It now works ok with one hand (heavy glove and cold water).
By the way I also bought another shut off last year to see if it was just an anomaly and low and behold it was not any better. So just split it, clean all parts, and relube before reassembly.
See you soon on the SeaTurtle.
Gabe