We were able to drive at highway speed for about five minutes or so,
the car would sputter and die. After it sat for a moment, (sometimes
little as 20 seconds) it would start right up and go again for about
another five minutes. The longer we sat, the longer we could drive
until it died again....
... It seems to idle with just a slight misfire, as long as there is
no load on the engine....
I had the exact same problem in my company car (not a triumph,
unfortunately). Turned out the fuel pump was bad and could not
maintain pressure. I'd check there.
looking for another one...