Anyone try the demo yet? I grabbed it off Xbox Live and gave it a spin. You can pick who you want to play as for a fight between Chuck Liddell and Mauricio Shogun Rua. I tried both, and won two pure striking battles (mainly because I couldn't figure out how to grapple). I beat Chuck by TKO when he turtled up as I punched him, and I beat Rua by knockout (he did a Flair flop).
Was fun. An improvement over the Dreamcast and PS2 UFC games I played long ago. Will be better once I figure out how to grapple. I know the right analog stick is involved, but I'm not quite sure how. Anyway, I'll probably pick this up at some point - it's out May 19th. Don't tell Kane.
I was looking up the release date just now, and Wikipedia tells me that the demo also includes a tutorial to teach you the controls. That might have been handy! I'll give that a try this weekend.
If you're still struggling with this and have held onto the game, maybe the patch they've just released will help? There's nothing specifically mentioning your problem, but maybe it's part of the memory leak issue?