Can't add much more to what has been said for the original problem, would go for noisy laptop PSU.
Got Dad a cheap USB wifi dongle and naff, guessing USB does not keep up and so slow. If a desktop would have go an interal PCI card with and external arial on a bit of wire or the one with a few fixed ones out of the back plate.
To your thing.
I got BT infinity and found the first 3 network connections holes 1 to 3 were not the best. I get the full 37.5Mb speed using hole 4, Think they are red. This is ok for you if on a home hub 3 supplied like I got last year.
No info given of set up so just a guess.
Can computer, guessing a hard to move desktop be moved and plug straight into the router, Does iplayer then go. If not then the netgear home plugs ( googles as not sure what the item was) are not wrong.
People have said the 85Mbs ones are not good enough so might be your internet connection speed gone up and they dont cope.
Try a speed test, what is it now. Mine does 37.5 on a good day.
There is one on here in the tools on home page.
Prove the homeplugs are at fault and not the internet by putting computer straight to router.
What I would do and work through things to see what ther problem is.
Is a HH3 then hole 4 is the one I found the best and got a 200Mbs home plug in and the other feeding another router as a switch to go to the telly, freeview box and blu ray ( gone are the days of it is just a telly, so complicated now!).
I have these and they come with a few 1m leads.
There are cheaper at £34 I think TP ones, just done amazom home plug search. There are 500mb ones and when speed goes to 80 soon and you can get more the 200Mbs might not do so worth getting 500mb ones now, If I get the faster internet speed then my homeplugs might not do and have to buy more so worth getting fastest ones now. Some people say the 200Mbs ones are no good but mine are but not massive cable runs of mains lead as just a small flat. Not running all over my country 40 room mansion.
Still if is your 85mbs ones prove to be the problem then get faster. 500mb might be worth the expense unless happy with 40mb internet and not upgrading anytime soon. Nice to spent money once, wander If I did right last year with the 200Mbs ones.