Are you guys connecting using wifi? I have the same issue when I try to race with my friend (I'm also using wifi). My friend has his ps3 connected directly to his router and says he has no issues with playing GT5 online against others. I need to get a long ethernet cable and test it out. Just throwing it out there in case you might want to try the same.
At this point it's more annoying than fun when getting disconnected during a race.