For some reason the GT5 server seems to be very glitchy with wireless connection, regardless of the signal strength. I had been using wireless on all other online games with no problems, but had been losing connection with GT5 online just as you described. I switched to wired connection and it seemed to help alot. There is also a 'cache' you should be cleaning out often, found at the GT5 main menu, next to 'GT mode' and 'Arcade', click on 'options' and 'clean cache'