I've fixed the problem! There were a few things I've rectified, and now the engine's running smoother than I've ever seen it.
I replaced the power steering pressure switch, 37 bucks from Repco.
I had a talk to a taxi mechanic, he told me that on the Landi convertors, the balance line (from the convertor to the airbox) didn't seem to make any difference. I described to him how my LPG system had been fitted, and WHERE the gas line was fitted to the air intake. Mine had a hole drilled in the rubber part of the air intake pipe, closer to the firewall. He showed me the spot next to the throttlebody where it should go. So off to the wreckers to buy another rubber air pipe section (40 bucks from the wreckers).
Once I made a new hole and moved the mixer, I went through the idle-setting process, then went for a drive. The engine was a lot more responsive, and felt better, but still had a high idle.
I went through the key-off diagnostics - all clear! But the engine running diags gave me the Map sensor out of range. So, with the engine running, I started looking for air leaks around the engine, with my trusty can of ether spray! When I gave a squirt near the firewall, the engine gave a rev, and I found a dodgy hose coming off the Map sensor.
Once I fixed that, reset the idle - I couldn't believe the difference! Just to be certain, I gave the ISC a good clean, too.
So thanks, guys! The suggestions you made got me asking questions, and looking around, and fixed the problem. Now I've got to give the car back to the missus, and get used to being back in my ole' diesel ute