Blind Pew
I was reminded by your bit about the DC10 nav problem and had a similar?problem on a southerly (overhead or close to Bermuda) track Heathrow to Miami - you may recall BOAC (or BA?) had a DC10 from Air New Zealand, also KSSU.
We had a large along track error - I noticed the time to next waypoint kept increasing - and went back to basics from VC10 nav days by decoupling and navigating by track and drift.
When in range of BDA it started to autotune and updated itself.
If I remember correctly the 3 inertial systems had some kind of averaging and were supposed to chuck out any one which the system suspected was in error.
I came to the conclusion that this wasn't happening and it was still averaging all 3.
I wrote it up and discussed it back at base and I think they came up with a change of software.
We had previously had an aircraft that failed to turn toward JFK when it got to Canada and overflew the turn by a couple of hundred miles (20 minutes or so) before Moncton? managed to contact the crew - they didn't know they were in VHF range.