If that problem is on an '84, then it is likely to be Digijet not digifant.
I had a similar problem with my '84.
It turned out to be the auxiliary air valve wiring which has a hot wire connection hidden in the main wiring loom and becomes energised at the same time the injectors and control box do, ie.when the fuel relay is energised. It was cutting off the power to the injectors when the aux air valve went short circuit or partially.  When it cooled off the  symtoms changed.   Is was a devil to find!
The schematics and books I had show nothing about the aux air valve being present.   It is usually used on CIS systems.
This may not be your problem but I thought I'd share it with you as the year is the same.
My solution was to dismantle the loom on the left side of the engine bay and separate the aux air valve hot wire from the injector hot wires and run it separately,from the other side of the fuel relay.
Good luck   Steve Cotsford