968 idle problem after 2 minutes

[Barry_L]

I have a '93 968 that has just developed a weird idle problem. From cold it's fine and idles nicely at about 1100rpm. Then about 2 minutes later, it drops to about 800rpm, and becomes extremely rough. Some time later the "EX" warning light comes on which relates to the catalytic converter (It's an ex-Japanese vehicle) A hot re-start is then almost impossible, but if it's allowed to cool down, a cold start is perfect, but then the whole cycle repeats itself. The 2 minutes between start and the problem occurring is pretty consistent.

Thus far, I have

Removed, cleaned and reseated all the sensor connectors around the engine front and intake.

The throttle pot measures fine and seems to work correctly.

The temp sensor reads about 3k when cold, and much less when hot.

The ISV resistance measures correctly, and vibrates with power on, both when cold and immediately after the rough idle starts.

Any ideas? I'm forever hopeful that somebody will say "Aha, that's the old "xyz" playing up

Thanks,

Barry.

[kfray]

Disconnect the O2 sensor and see if the symptoms change?