1984 B2 Coupe GT 2.2 fuel injection

[Steely]

A week ago, as I hit the motorway and 75mph, the car suddenly started to hesitate and lose power (no probs or symptoms before then) the symptoms now are when starting cold, it runs ok for a few mins, for maybe a mile or so, it accelerates well in all gears, then suddenly starts to hesitate under acceleration, gets worse and worse, occasionally it will stall when pulling away.

I turn it off and back on, it will be ok again for a minuet or so, then the trouble starts over again. It idles fine.

Fuel pump seems to be constant, filter is clean, petrol is good (even drained and refilled), plugs are good.. any ideas?