I suspect interference -- fairly broad so changing channels on the router won't necessarily help but it's worth a try. Better to try to relocate the router away from the TV.
If the ISP is a cable service (and the TV is cable too) could this cause a slow down ? I've never heard of this but who knows ?
I have the TV and the Router in opposite sides of the house. However, when I switch the TV off, the network runs at top connectivity - almost instantaneous. I've tried to change the Channel to 11 (recommended for OZ), but couldn't on the D-Link it stays on channel 1 (perhaps theres som ehelp out there to chance channels on the D-Link).
I also have an Ethernet connection on the D-link as well; and that link runs OK. So I think the Bravia interferes with the signal.
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The D-Link is probably set to select channels automatically. Personally, I don't care for the complexity and inconsistency between models of D-Link's menu designs.
I was going to suggest that the Sony may be producing interference on the mains circuit that's affecting the wireless -- if the issue is also affecting wired connections that idea has even more possibility. There could be a fault in the Sony's power supply. If you can borrow a similar type of TV (any flatscreen) see if the issue goes away when you substitute it for the Sony.
You didn't answer whether the ISP is cable or ADSL.