My experience in owning a Regina 940 for 19 or so years is that the CPS will rarely if ever throw a code. Replacing it when suspect is good practice as it can shut you down. The radio relay can be a problem, though I've never had it, replaced it a couple of years back as a precaution when I had the exact symptoms you have.

You need to check/replace the main fuel pump relay, that is usually the culprit. It gets hot and the solder joints crack. Pull the relay, pop the cover off and check the solder joints with some magnification. The 3 big ones usually crack. I had 2 relays and both had cracks in the same locations. Re-solder them and you will be back in business.

I have found the coil amplifier to be dead nuts reliable. I have 2 of them also, bought 2nd one when I was chasing the same problem. I have cleaned the connection of the coil to amplifier like once and put a small amount of conductive grease on it. Currently running the 1993 one.

Of course, your mileage may vary, but I'd check the relay behind the ashtray first.

Mike

Also check the grounds. I had a similar problem and cleaning the grounds cured it. The ground connections are on the fuel rail and on a connector to the body on both sides of the engine compartment up by the headlights.

Good luck.
Phil

Have you read out the fault codes? A faulty CPS or it's wiring "should" record a fault code. A bad RIR won't record a code, but will interrupt power to the injectors. I believe the FAQ describes a way to jump wire the RIR in a way that will provide constant power to the injectors (for troubleshooting purposes only).

Another part of the Rex/Regina system that can act up without recording a fault code is the combination coil/power stage. The internal connections and mating surfaces become dirty and/or oxidized which will interrupt the ignition, especially when it gets warm. Removing a couple torx screws will provide access to the internals so you can clean it up.

So NOW when it craps out, does the tach still drop to zero? The CPS, or its wiring, are about the only things I can think of that will cause that. There has been discussion in the past pertaining to a white band or yellow band on the CPS. Apparently the cars were built with one color that is notorious for failing so you want to replace it with the other color. Sorry but I don't recall which is good and which is bad so hopefully someone else will know. Did you replace it with an OE brand or an aftermarket version?
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)