RunningMan:
Actually, just re-reading, you tried another filter, and it didn't help, so unlikely to be a faulty filter.
Given unplugging and reconnecting everything has made such a dramatic improvement, it's likely something you have disturbed while doing this - either an intermittent loose connection or something corroded that cleaned up as it was moved. Check for loose wires on the jackpoint, moisture and oxidisation on the the plug and sockets contacts - they should be bright gold colour.
But right now the modem is connected directly into the wall jack point with the cable that was on my landline phone. That is what made the net go 'normally' again. Before I realised how to do this (taking cable off phone and using it) I had the original cable from the modem plugged into the line filter..... and then into the wall as that is the only way that will work. Maybe it is that actual cable that is faulty.... the one that was in the modem and then into the line filter ?