Super weird problems to have, but a neglected 22 year old 911 with nearly 260,000km will be that way I guess.
Anyway, this morning removed the airbox, belt and alternator (this time only took 15 minutes now I've done it a few times!). Unplugged the electrical connections from the alternator, and reinstalled everything.
Car runs fine, perfect idle, not a problem. This isolates the problem to the alternator/charging circuit, and everything else is good.
Removed alternator again, and used a spare old voltage regulator (I'd put a newregulator in this alternator in 2018). Refitted everything, and same old problems arise - low revs, misfires, backfires, lots of motor rocking, eventually stalling.
While the alternator was out, I rechecked the new Y cable (all connections appear good), checked all vacuum connections (again seem fine).
Next step? I'll probably remove the alternator and send it to an auto electrician for testing. Possible issue - alternator has some crazy internal fault that only shows up at idle? Voltage fluctuations, other things causing engine to run poorly. Hopefully a good auto elec will be able to bench test, simulate load etc. Then get alternator rebuilt, reinstall, and hope it works.
Was thinking of selling the 911 once all these issues are fixed - but I still like driving the car!