'87 Turbo Stalling

[USAndretti42]

Someone I know has a 1987 Turbo with K-Jetronic injection (Californian model). It was driving very nicely but last week it started stalling at idle.

I drove it at the weekend. It starts very quickly from cold but, as soon as you let the engine come to idle, it stops. It's the same cold or hot. You can catch it if you open the throttle again. Sometimes, the car did idle but when it did, the engine ticked over at about 1500 rpm. There didn't seem any rhyme or reason why it sometimes ticked over, albeit at high speed, and other times it stalled.

Originally I suspected the extra air valve but that wouldn't explain why it sometimes ticked over and sometimes didn't.

Before it was fully warm, the engine seemed to be misfiring below about 2500 rpm but I didn't notice this when it was fully warm.

Any ideas of what it is or what I should look at first?