The symptons make me think duff lambda sensor - when you start the engine hot or cold it'll run in open loop mode for a short while (until a certain temp is reached if starting from cold, or for a given time if the engine is already up to temperature), then switch to closed loop.
If the lambda signal is duff, then when the engine switches to closed loop mode, the ecu will try and correct the fuelling on the basis of the duff lambda signal and struggle. If the ecu is unable to correct the fuelling then I suspect the ecu will revert back to open loop as a fail safe.
As a quick test, put a voltmeter on the lambda signal wire, and see what it does - it should read around 0.8v when you first start the engine, and then once up to temperature, it should switch between 0.3 v (lean) and 0.8 v (rich) at roughly 1Hz (at idle, and faster with increased revs).
Food for thought....?
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