Looks like getting a new modem/router is a good idea. The netgears seem to have a good reputation and can hang onto the Infinity lines OK (again from the BTCare forum postings). The Draytek does seem to be the modem/router of choice.
Many have found that just a new Netgear router to replace the HH3 has given much more stable connections and good speeds- a number of favourable posts around.
Many have seen the "slow down" problem (myself included). You start off with a good speed e.g. 37 megs on a 40 line, and over maybe a day or so it drops to single figures. A HH3 restart, with a new IP, seems to solve it in many cases. It seems to be a generic problem, and no-one at BT wants to own the issue. That said, now I am on 80/20, and have installed homeplugs for my machines, I see 56-64 megs without fail and stable with no restarts required (IPP 66.2). The only difference being the new speed and NOT using the HH3 wireless.
Good luck with the new kit when you get it!
1999 Freeserve Dial up --> Freeserve Anytime ---> 2005 Wanadoo 512k BB ---> Jul 2009 Orange up 8 Megs(hah!) --> Nov 2009 O2 LLU -> Nov 2011 BT Infinity( getting 64 megs now on ethernet via TPlink homeplug 200's)
Dual Yamaha AW16G user using MIDI MMC for 16 tracks
FujiFilm HS-10 User