I see that this thread is a couple of months old but, were you able to solve the problem? I had the same problem with my Volvo Penta 5.8fi. If the engine cuts out at 2800 rpm, it is the Slow Circuit being activated. The ECM will stop firing injectors until the rpm drops below. The ECM does not remember this mode. Once the sensors return to normal, it switches this mode off. It only works for a low oil pressure or over heated water temp. and they are controlled by seperate sensors. The engine actually has two oil pressure sensors and two temperature sensors. The sensors for the SLOW circuit connect to ground if their threshold is met. All SLOW circuit sensors are connected to the same wire that goes to the ECM. If any sensor is grounded (or if the wire has a break and is grounded) will cause the ECM to enter the SLOW mode. The ways to diagnose it are to use a code scanner and plug it into the diag port for the ECM. It will give a code indicating that the computer is in SLOW mode. (I can't remember which code it was, it was in the Volvo Penta service manual). You can also test the wire that goes to the computer to see if it is grounded. (At the back of the engine where the wires come out of the harness for for trim/tilt sensor, there are two loose wires. One is ground and the other (I believe it is brown) connects to the SLOW circuit. If you jumper these two wires, you'll put the computer in SLOW mode. You can test the brown one by itself to see if it is grounded.) The next step would be to disconnect the oil and temp sensors seperately to see which one is not functioning and then replace it. For me, it was the oil sensor. Even though the oil guage was showing proper pressure through its sensor, the oil sensor for the SLOW circuit was faulty.
Something most don't know or think about is that the ECM will go into "slow mode" if the trim indicator is not working as it should. (They didn't want high RPMs if the drive was up.)
Is your trim guage working properly?
The ECM is powered up via a separate lead with fuse that is attached to a battery and it’s always on. To power cycle it remove the fuse or disconnect it from the battery post.