The first thing to try, if you haven't already, is a new filter. ADSL2+ uses a wider frequency range, so your filter may not be adequate. Do you have a test socket, ie can you remove the bottom part of the faceplate on the master socket? If so, do that and connect to the test socket which will be revealed. If that cures the problem, then your internal phone wiring is to blame.
If you get a BT engineer out, and a fault is found on your side of the test socket, you are looking at a bill of £200+ these days.
If you do revert to an 8M service, the line is simply remodulated to use the narrower spectrum of tones, and you will keep your higher download allowance.