Sorry that you and your friend are going through this nightmare. What you are experiencing can be the issue with repair shops . . . they too are human and can be convinced the problem is one thing, only to find out that it is not. Meanwhile the repair bill adds up. The DIY approach has its challenges too, but you are not paying yourself $100/hour to do some trial and error.
Anyway, Please, Please go through the Mercruiser manual for this engine, regarding the fuel pressure. Do not rely on hearsay. I had seen/read 37-43 psi of fuel pressure for the MPI engines, but when I looked in the manual for my engine. . . the spec was 34-38 psi at the rail, and the pump should be providing more than 38 psi, but not over 60 psi. There may be an engine that spec's at 37-43 psi, but it is not my engine, and probably is not your engine, unless you can verify it in the Mercruiser manual for your engine.
Anyway, I just want to re-cap the background and symptoms of your friend's boat/engine, and advise us if things have changed as a result of all this work. . .
1) new to him boat, ran well at the sea trial, the boat is 19 years old, had sat for 5 years before he bought it. Generally, a very low hour engine for its age.
2) sometimes runs well and sometimes does not. Mostly does not.
3) when it is running poorly it does not have much power, tops out at about 3800 RPM 35 mph. When running well it goes to 5000 RPM & 60+ mph
4) some effort on the fuel system and ignition system (filters, sea foam, dist cap, rotor, module, etc.) to no avail.
5) replaced the 'cool fuel' pressure pump and fuel pressure regulator - no improvement
6) shop replaced the 'coil' which is at the base of the distributor (not the ignition coil) - no improvement. Fuel pressure checked out at 35 psi per their testing. No fault codes on the ECM.
7) Now what :noidea:
Is this pretty much correct?
Looking at all this, I still go back to thinking about the 5 year lay-up and the effect that it could (probably did) have on the fuel system. I am wondering what things look like internally within the fuel rails?
Was there any ECM (MEFI) data taken from the engine during operation? I believe it did not have any fault codes.
My hunch is that there may be some 'jelled' fuel in the fuel rail as a result of the 5 year layup. You could try to purge or even disassembly/clean the fuel rail(s).