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)