Don't be fooled by that bugger of a game. It's a great game but when I exchanged my copy of PGR I thought I'd solved it too. It came back. I exchanged consoles because of it, just like you, I thought I'd kissed the problem goodbye, but it came back.
Maybe you've been one of the lucky ones. If not, be sure to post back when it starts again and try not to break anything out of frustration. You could eat the disc though, that'd be okay.
TUUK