What Isp are you with? If it is AOL change the router MTU to 1400, if not try it anyway.
I think the most likely cause is the adsl signal. Is everything microfiltered using good quality microfilters, telephones, faxes, ans machines, alarm, sky ?
Is there any DIY telephone cabling, you could try it back at the BT mastersocket to rule out cabling within the house.