Have you tried online mode? Even worst than Forza 3's online, seriously, this has to be a joke. A lot of lag, it freezes a lot of times and the worst part: ****** filters and options: karts, F1 or all cars. No Nascar, no WRC, no Super GT, no sports car, no 'less than X HP cars', nothing. If you are the host you can filter cars... one by one! you have to select all cars you want to allow, and you cant save your settings. No Qualifying session, you can even disable penalties...
It's nonsense. PGR2, a 8 years old game, is much better than this one. GT5 needs a MASSIVE update, this is not about lag or some glitches.