Idle / engine problem 1990 Coupe 3A 2.0

1990 UK Coupe, 2.0e 4 cylinder 3A engine, 140,000 miles.
The engine idles fine when started, barring the occasional stall initially. Engine pulls great, feels smooth when applying & releasing
throttle. Economy 32mpg (UK) about town, 38mpg on motorway, which I reckon is pretty good. Stopping at lights, the idle is around 800 revs, a bit lumpy but basically ok. Temp is about 1/3 up the guage, or 1/4 on motorway.
After driving for a 'while' (anything from 5 mins to 30 mins) the engine mode will change - applying or releasing the throttle makes the engine jerk, and when stopping at lights, the idle will bounce between 1000 & 1100 revs, once a second. These two symptoms are always concurrent. The economy is the same as before. This switch in modes seems unrelated to time, or temperature. It often follows a sprint of heavy throttle / acceleration. The engine will stay in this mode until it's shut off. when restarted, the first (smooth) mode occurs, and the cycle repeats.
I've changed the ISV, without major difference. I also found and repaired a bad vacuum hose, which helped the 'good' idle a bit, but didn't change the second 'bad' engine mode.
Any ideas on where I should look next?
Many thanks, Jamie
Any ideas on where I should look next?
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It's not the culprit but you should change the thermostat when you've got the time. The readings are way to low.
Ronald
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snipped-for-privacy@gmail.com wrote:

Exhaust gas recirculation (EGR) valve if your car has one? Any fault codes reported by ECU?
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Jamie, The oscillations in idle sound like a problem with the oxygen sensor - if it's the original in the car it should be changed by now anyway - the Audi OXS may be expensive, but many have found that a generic Bosch one can be spliced in to the Audi harness at a fraction of the cost. Also, have you tried pulling error codes to find out if any errors have been logged by the engine management? Cheers! Steve Sears 1987 Audi 5kTQ 1980 Audi 5k 1962 and '64 Auto Union DKW Junior deLuxes (SPAM Blocker NOTE: Remove SHOES to reply)

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