Mine also like that, DC after a few minutes everytime. Seems like the router detect the PS3 is in idle then DC its connection. I dunno, just my speculation.
So I bought a new router and use that instead. Below is what its like :
[TM fiber whatisitagain] --> [TM router] --> [My router Belkin] --> [PS3 in DMZ setting, wired]

My TM router is using google open DNS.
This settings never give me DC anymore You guys should try. The downside is you need another plug hole for an extra router...

I've never experience such problem. I am not using tm router btw and my ps3 is wired.
My setup is like below

TM modem (dunno what its name) >> TM router >> My belkin router >> PS3

TM router is needed because i need it to connect to set top box for IPTV