car stalls when clutch pressed down

Try resetting the ECU again, and just let it idle for at least half an hour on the first start up before even touching the throttle.

Then take it out for a careful drive for a few miles, before finding a quick road and opening it up.

If this doesn't work, try cleaning the idle air control valve out.

PS- just going on my own experience with my ST24. Whenever I've had to disconnect the battery for whatever reason I've always had terrible problems with stalling when pulling up as the ECU re-learns unless I let it idle long enough to warm right up and some more. It's like they find it difficult to get the idle right or something? I've not really heard of many others with similar problems but I can replicate it on demand with mine.

I'd make an educated guess that it runs ok when cold because the system is running closed loop and on a default richer map. Again, when mine isn't quite right after an ECU reset it behaves the same.

The hunting shouldn't happen - maybe the dual-mass flywheel is a problem?

When you press the clutch the whole gearbox train is removed so rotating inertia is reduced.

The fact that it idles OK normally makes me suspect it is not control.

If you give the throttle a blip to take the revs up to 3000 and let it fall, does it float down to 750RPM and stay there, or does it drop below that and come up. If it drops then I agree that the air bypass valve may be sticky or blocked.

Paul.