OK, I know it's been a while but I thought I'd better report back on this one since it seems to be a pretty common problem.

I followed all the advice in this thread and elsewhere. I did:

1. Airbox out, clean injector - problem marginally improved but still there.
2. Airbox out (again), adjust TPS (THAD/APAD values) - all OK, no improvement.

Then, while re-mounting the airbox and preparing to break out TuneECU yet again I noticed a gap between the (Evo 1) lid and (foam) filter on both sides. No idea why I didn't notice this before. I pulled the filter and the inside of the airbox was clean so I put it back together and ran a bead of silicone around the 'box/lid joint. Waddaya know? Problem solved! Constant idle speed, no choking or stalling!

So, too lean at low rpm apparently. One more for the checklist.

Many thanks for everyone's help!