I just wanted to update this thread to say thanks and that a combination of issues were addressed over the last few days, which have not only resolved my problems but dramatically improved the whole feel of the car. I cannot say exactly which issue was the main culprit. Most likely it was a combination of several of them.
We still are not sure why the idle switch being disconnected on Ken's car causes no issues for him but clearly leaving it like that is pointless if you retain an ISC so thanks Andre, we have addressed this with a four wire TPS. we swapped in another ISC which smoothed things out but didn't stop RPMs dropping under load. So we then swapped in another alternator and an HKS grounding kit that I picked up from Cory together with some heavy gauge wire to run from the alternator to the battery. This stopped the rpms dropping by more than a minimal amount. A new battery might be a good idea!
As mentioned in the last post, the power steering sensor was also clearly not working. Again I don't think it was the cause of all the issues but it undoubtedly contributed significantly to the stalling issues I experienced during parking or maneuvering at slow speeds.
Another radiator was also fitted but that was a separate issue with the radiator cap not sealing and spewing coolant the whole time. I was just getting tired of adding coolant and cleaning up the mess.
So, I'm very pleased at present. I'll see how things go over the next few weeks and if any issues come back but I think in general the electrical system just wasn't up to scratch and as usual a few 20 year old parts had just given up the ghost.
Edit: Just wanted to update this and state that the idle has remained rock solid since I last made this post with no new idle issues whatsoever. So in my case these issues were corrected by the replacement of the power steering sensor (to bump up the rpms) and the fitment of the following: -
(1) an older brown coloured JDM ISC (part no. unknown);
(2) an EVO III 4-wire TPS (since the S90 throttle body had no TPS switch);
(3) a working higher amp alternator (plus a thicker gauge charging wire from the alternator to the battery); and
(4) an HKS grounding kit