After travelling in excess of 60 miles at 75 mph, the engine tone suddenly switches to a lower level and there is a substantial loss of power - only achieve 2000 rpm on level road. If one then switches the engine off for 5 minutes, it resumes normally for another 5 to 10 miles before exhibiting the same problem. Any ideas where I should start fault finding?

 

I had an issue a bit like this on my E it was the coil....

 

If it is not the coil, it could be something fuel related. Could be a clogged fuel filer or some garbage clogging fuel flow from the tank.

 

Hi, It sounds to me like the ignition amplifier is overheating. Or the coil is overheating. but when mine did that it was the amplifiuer. I ended up replacing my distributor and amplifier with a kit that puts more modern internals into the original cases.
It has been much more reliable. Been in the car now for about 6 years.