I have raced with the same 50 odd people for a year and a half. After 100s of official competition races and probably 1000s of casual races with the same group it is always the same people with the connection problems.
Admittedly GT5 Net code could be better and not so sensitive to router type and set up, but the majority of the issues people have are down to the personal connection, router type and router set up they have. Despite this people love to blame PSN and the mythical GT5 servers. PSN and the Mythical GT5 servers are more or less the same for everyone but when it comes to personal internet connections, routers and router set up, there are a magnitude of Variables that all influence the end result you get when playing online.
I live in Italy, first connection I had was with an old Phillips router. Although old it was quite good and had tons of settings. When I first started racing in my group, I had many disconnects, could often not see or hear other racers and also my car glitched around. It got so bad people did not want to race with me. 5 minuets of router PS3 set up and problems fixed instantly.
Moved house a few months ago, and got a new connection. Could not use the old Phillips router so took an old Cisco from my GF's uncle. Out of the box and on default settings, once in Private lounges racing with the same guys, I think I go disconnected around 8 times in a 5 hour stint of online play. 5 mins later, quick router/PS3 set up and now I am back to a stable set up.