Best explanation I found, with acknowledgment to swedishbricks.net:

Radio Suppression Relay. [What is the Radio Suppression Relay and where is it?] The radio suppression relay (as it's called) is in actually a fuel injector relay. The pre 1986 injectors were supplied constant + power to one terminal and supplied a timed ground to the other terminal via the ECM. The constant + power used to come from the fuel pump/injection system relay. In 1986 Volvo decided to isolate the fuel injector circuit because of some radio interference created by the pulsing circuit. The solution was to energize a relay (the radio suppression relay) with the pump/system relay feed instead of using it to power the injectors directly. The radio suppression relay then supplies the + power to the injectors. The timed ground still comes from the ECM. The reason Alex can't find his radio suppression relay is because it's located on the other side of the engine compartment behind the power steering reservoir, (relay closest to the engine of the two relays mounted there Alex). For the most part the non-turbo engines had the radio suppression relay mounted on the right side (pass) of the engine compartment on or near the coolant reservoir as Alex mentioned. The turbo engines had the radio suppression relay mounted on the left side (drivers) of the engine compartment on or near the shock tower. Not set in stone just as a general rule. The radio suppression relay was used from 1986 until 1995 on almost all LH injected 4 cyl and V6 engines. There may be a couple of exceptions. The 5cyl and straight 6cyl engines do not use one. When it's bad or missing, the radio will work fine, but the engine will not run. Remember even though it's called a radio suppression relay it's function is to supply + power to the fuel injectors.
Problem diagnosis: If you suspect that your radio suppression relay is faulty (causing hot start problems, etc.) try switching the leads with the electric cooling fan relay if your car is so equipped. This relay is identical and in many car lines is located right next to the radio suppression FI relay. If your fan stops and your engine starts, then you've isolated the problem.

Yeah, I did some reading and found that too. I'm not convinced it was my problem, though... I thought it was at the time but now I'm not so sure.

I had troubles with it after I took it to Lethbridge last time too, and I thought I knew what it was then. It might just show up on long road trips - ignition power stage is one idea, dying fuel pump relay is another, and I know my fuel pressure regulator isn't healthy (but TOUGH to find).

I'm driving it and it hasn't hiccuped since 12km south of Nanton, so I'll let it ride and see what happens. It's likely all going to change this winter anyway...

^ The one comment I would add from my experience with the Valdez is that swapping the a/c fan relay isn't as foolproof as they suggest on SB, in my case it was the downfall of the shop that couldn't Dx the problem b/c they swapped a bad fan relay with the same no start. I think it was a post I found on BB that said to jumper the two fat wires (power) on the RSR plug as the definitive method to exclude the relay and keep looking elsewhere, in the end it was the "a ha" moment in my troubleshooting adventure.

Yes, and in re-reading your posts I realize that the RSR wasn't actually your problem, until you swapped it for an unknown one which was bad. Then once you fixed the grounds the new/bad RSR came into play...

I actually think my problem is in a failing in-tank pump, maybe, or a dying fuel pressure regulator, or pump relay, or something like that. At one point I removed the hose from the fuel pressure regulator (with the only tool I had in the car, a pocketknife) and nothing was coming out though I'm pretty sure I could hear the pumps over the noise of highway 2. It was running fine for an hour before and then it started to die (would run normally, lose power for a second, run normally for a few, lose power for another second), and got worse and worse until it stalled. 5-10 minutes later I could fight it back to life and it would run for a while. Happened 2 or 3 times. I swapped RSRs and nothing happened at first, and then after swapping back and forth a few times it eventually ran all the way home (and continues to run) which made me think it might have been the RSR until I looked up what it actually does. The fuel tank was just below half indicated when it started happening (only put 48.5 litres into an 80 litre tank when I got home - indicated empty leaves me with almost 20 litres in the tank, but I don't know where it stops pulling fuel.)

If anyone's heard me try and start my car, you know how much cranking I've got to do. I know I've got a fuel delivery / fuel pressure problem already, but it's never caused me much grief besides hot starts.