Problem with gran turismo 5

[element_qs]

I've been doing the superspeedway - Indy race in like the wind and I've won this race over and over just farming credits. Went on it today and suddenly I cant win the race because the game has messed up somehow. I used to be able to corner at about 210-220 mph with my set up and although my set up has not changed the car cant even take the corners at 180 mph now its changed the tyres to hard sports tyres even though I had soft racing tyres. It will not let me change them back and the cars acceleration has dropped its like I'm in the worlds slowest car compared to them. Now does anyone have any suggestions on how to fix this?
Thanks.