I had this problem in an old Escort of mine. Like you said, it came on when I was going around corners and then it started coming on whenever it wanted to. It was because something was shorting out. Maybe that's what's going on with yours.
I hate my dealer. They use Enterprise for their loaners and last time I brought miata in I got a bone-stock automatic mitsu lancer. I swear the clerk did the head-bang as I left. IF I ever go back there it'll be with a fully-loaded semi-auto shotgun, and I'll use it if I get anything short of an SVT Focus!!