Hi Paul: It does not sounds like a CPS or flywheel sensor to me. My logic: When one of those is failing, if it runs at all, the car will stumble like the key has been turned off and back on. This is because when the Marelli ignition computer looses CPS or engine speed inputs, it stops firing the coils, and it also stops sending a signal to the Lucas FI ECU. So everything is shut down. Due to the loss of signal to to coils, the tach drops quickly drops to zero.
I would at least inspect the cap and rotor for any signs of arcing. While I don’t think this is likely, you could be in the early stages of the “Marelli meltdown” where you are losing 6 cylinders due to rotor arcing. Obviously check the wires to the plugs, to both coils, and to the low voltage side of both coils. I hate to suppose, but it seems to me that since it runs at high RPM under light load that you don’t have an ignition problem. Most ignition problems get worse at higher RPM.
Assuming the cap and rotor and other ignition components look good, I think you are on the right track with fuel starvation. More throttle just means more air, then the FI is supposed to keep up from there. So it sounds like you are giving it more air and it’s not getting enough gas to go along with it.
You’ve already done the obvious with the main and sump filters. Maybe check the fuel pressure regulators, particularly on the left bank, for any signs of fuel leaking from the vacuum diaphragm port, which indicates a split diaphragm which probably means very low pressure at the rail.
Could there be water in the fuel? Maybe a can of Heet or other stabilizer to break up the water. That stuff is basically alcohol which is not great for fuel systems but one can won’t hurt if you run through a tank of gas over the next few days.
Ideally you want to get a fuel pressure gauge on there to get some idea of the health of the fuel pump. You can easily t into the main line feed on the right fender by the firewall, it’s just hose clamped on.
You didn’t mention how the car had been in the months leading up to this. Driven regularly? Other work recently completed? Other “unrelated” symptoms?
Others more knowledgeable about FI problems than myself will surely chime in.