You mentioned "FPR tested, not leaking" -- What did you mean by leaking? Doesn't dump fuel into the intake by it's vacuum hose, or that the system holds pressure after shut down for 20 minutes as specified by Volvo? Both are valid concerns.
Also, it appears after reading over this thread you make no reference to the RPM sensor or knock sensors, both inputs to the Motronic 4.3 ECU.
knock sensor problems are supposed to throw a code, and to my knowledge anything but a common failure, but it wouldn't hurt to check the wiring and clean the connectors. these can't be resistance tested as far as I understand. the KS does cause the ECU to ease timing and dump in more fuel, so... at least it's on the lines of running weak and rich.
The RPM sensor, when malfunctioning, usually results in a no-start or random stalling/ignition dropout, and the tach will jump/jiggle or drop to zero (as the RPM sensor feeds the tach) -- but with all you've already been through, it wouldn't hurt to inspect the wiring harness to it and clean it's electrical connections. You should have 260-340 ohms across the terminals. The RPM and CMP sensor work together to allow the ECU to determine the crankshaft position and working cycle.B5234T/B5204T/B5234T5: If the knock sensors detect that the engine is knocking beyond a specific threshold value, the timing will be initially retarded and the fuel/air mixture will be subsequently enriched to eliminate knocking. If the knocking does not stop despite these measures, the control module will reduce the boost pressure until the knocking ceases.
One other component that's critical that you haven't mentioned is the ignition coil. While these aren't common failure items, pretty rare actually, a weak coil does often show it's worst at high RPMs, as the demand on it is much higher. Inability to rev past a certain RPM is not uncommon with ignition coil failure, usually caused by dielectric failure in the coil and resulting internal arcing. However, it's usually accompanied by misfiring, rough running, sputter, or other symptoms of intermittent spark.
These three items are all unlikely, IMHO, to contribute to the problem you have described.. But since we're out of ideas I figured a look at the list of inputs to the ECU would be a good place to start -- process of elimination.
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