I had this problem, too, and fixed it:
I was using a D-link DI-524 wireless router (wired connection to PS3) and a Motorola Surfboard modem. I had tweaked everything possible on the router in favor of PSN and GT5 in terms of ports, DMZ, everything, and nothing seemed to work.
So I strolled on over to Fry's and bought a new router and a new modem. The router I chose fixed the problem (I took the modem back) - it's a DIR-655. It has QOS and many features that the DI-524 did not, but awesomely enough GT5 works perfectly just plugging the router in and going with it. I have since QOS'd the PS3's IP and forwarded the ports needed by PSN and GT5 just in case, but I never had one disconnect even while the router sat in its default configuration.
Hope this helps!