Go to,
Workshop manuals, owners manuals and service protocols for your engine. Get your manual and service protocol here.
www.volvopenta.com
and use your engine serial number to download a copy of the operators manual for your engine.
Read page 15. do you have an engine status panel installed in your boat? Do the lights light up with the key on? Do you hear the 3 beeps from the warning horn?
You may now be in power reduction mode. This engine will limit the RPMs to 3500 or less when that happens.
Oil pressure needs to be above 5 psi to stay out of power reduction mode.
Fuel pressure should be 15 psi at the low pressure test port on the fuel pumps and 50-60 psi at the test port on the fuel rail. Yes you should test both these pressures at key on, running at idle and at wide open throttle UNDER LOAD. do not run this engine over 1500 rpm without a load.
50 psi might be a little low. Is this a quality test gauge your using?
This is a catalyst engine. They don't last very long if you run them when they have issues. Highly suggest you find and fix whatever is wrong with this engine.