I know there are a lot more SH4 vets around here that can give you a better idea of a game-specific problem than I can, but from the viewpoint of someone who has spent the better part of their life as the nerdy gamer sort, it sounds an awful lot like it was just a repeating keystroke, specifically whatever you have as your 'TC Down' key. Every once in awhile, an application will get terribly confused and continually think a key is active, and my first impression would be that SH4 is convinced you're constantly mashing the - key. If this is the case, restarting the game should fix it, if not, try and hit the - key (by default, or whatever your TC down input is if you've rebound it) once or twice in-game to see if it registers a key release.
Someone may come along with a specific problem/solution they've encountered in the game before, in which case I'd defer to them since this is all coming from my experience with other games, but it's entirely possible it was just an application hiccup. Hope you get it worked out either way!
