If you have a laptop you could put in the barn for a while, I would install Wireshark on the laptop and see whether that shows you any errors and what sort of errors. If you do see errors then try connecting the laptop to the ext cabinet and see if you get errors from there to your DSL router, if so move further along trying to identify the faulty segment.
Poorly terminated IDC connections in cabinets and wall plates sometimes only show up after a prolonged period especially if temperatures change. Worth checking the IDC connections in these locations if a cable that uses them shows up as suspicious.
Ethernet Interfaces can sometimes fail with varying symptoms, so if all the cabling checks out fine, then try a different port on your DSL router and a different Ino shield just to be sure.
As others have said, if you have any CCA cable be very cautious, it is often much cheaper than proper copper cable but also more prone to error. The aluminium in CCA can also corrode more easily especially where the copper coating is damaged (IDC connectors, screws etc.), this is more likely if the atmosphere is damp or colder.
Is there anywhere wildlife could attack the cable, mice like to chew the insulation which can cause an opening for other problems. Wasps can also damage the insulation , though I have not seen that go through to the metal cores yet.
I have used ethernet at 1GHz made up of about six patch cables (CAT5 - each 1 or 2m long) joined together plus patch cable from repeater to cabinet and cabinet to wall plate (cat 6) without problems, the total length being around 30m - none of it was CCA. I would not expect your cable length to be an issue, especially if it worked fine for 12 months.