A things:

1. Switch off mains in your property at the main fuseboard and see if performance is better
2. If yes, then switch each fuse back on to help identify which circuit, and then you can go through all the kit on that circuit
3. Are you using master socket for the ADSL, with an ADSL faceplate, or IPlate or have removed the ring wire? All things that can make the ADSL less prone to interference
4. Buy a proper twisted pair RJ11 lead for modem - may help a little
5. Use an battery powered AM Radio to try and find the source of noise, i.e. tune it off a station a bit and see if the noise is worse in one particular area
6. The improved speed outside of a powercut may be down partially to less cross talk from other ADSL lines in the area, i.e. power goes off 98% of modems will then be off crosstalk vanishes, and if your estate has a very high >75% takeup of ADSL it can have an effect.

I fear, Opti, that you'll not get too much help here if you adopt the shirty attitude "Do I have to find the exact post just to satisfy you?".
Most people on the TBB sites are friendy, and such a brusque attitude will get you nowhere.
As an aside, unless you're resident elsewhere than the UK, then METRES is spelt just that, not METERS.

"METRES is spelt just that, not METERS"

It is. I've got two under the stairs

John.

Yes John, but how many meters - gas & electricity - standing side by side, is it before you reach the nearest street cab.
Cheers, Les.

Edited by lelboy (Sun 16-Aug-09 16:35:28)

"nearest street cab"

There's one parks up overnight about 4 doors away

John.

Do they let you have the manure for your roses?

Yeah. In exchange for some oil for their lamps.

John.

They must be hansome

You don't see many green ones with a wheel at each corner - although the owners of both sorts spend their time ripping us off...
Les