It has nothing to do with updates, it works perfectly for me when UPnP is turned off - I can get UPnP no problem, but that causes other issues for me due to the router that I have.
The problem is normally with the router in the case of garbled downloads. UPnP would do nothing to solve that, all UPnP does is open ports automatically rather than you having to port-forward. If you have a router that doesn't agree with the PS3 (I have one), then the problem can be solved (as in my case) by turning off UPnP, and placing the PS3 into the DMZ. Another solution is firmware updates on the router, or buying a new router. The problem occurs mainly with ISP routers supplied for free with custom firmware on them, or in my case, a specific model of router that is know to have issues with the PS3.
Zub, what make and model of router do you have?