Not a very scientific method of checking, but have you tried plugging a telephone handset into the line(s) used by your modems, and listening for any noise, interference etc?
What about checking the plugs, sockets and cables used between modems and your outside phone line connection. Line plugs and sockets can get dirty and corroded over time, and this can cause the sort of problems you're experiencing. Has Mrs.Mopp run over the cable with the vacuum, and damaged it?
If you're not plugging the modem cable directly into the master socket, i.e. you've got extension wiring etc., try temporarily moving one computer so you can plug it in without any extra cabling. Does this improve things?
ROGER - G0AOZ.