04 Monterey 5.0 MPI power loss

hello, new poster here.

I have recently purchased a 2004 5.0 MPI Monterey. On first time out it ran smooth but seemed lacking on power. I am a pretty good auto mechanic so I performed a tune up on the old 305. Plugs, rotor, cap, wires changed fuel separator, even changed the coil and module assembly. Took boat back out, ran great, until I ran full out for about an hour, then it suddenly dropped about 1000 rpms and ran like it did previous to the tune up, lacking power. idle remains very smooth, acceleration is very smooth, just lacked power. After putting around for about 15-20 minutes I took off and all was good again for about 20 minutes at wide open, then the sudden drop again. I have now looked in the fuel tank, it is very clean, checked the anti siphon valve, it is not stuck and appears to be working fine. any suggestions would be much appreciated. thanks

Not one response, did I do something wrong??

you may be going into "Guardian Mode" from another issue. No you did noting wrong ,its that were not on here every waking minute to give an answer

Are you anywhere near a Mercruiser shop that can plug in and check for codes?

Not real close, but that's not out of the question either. I was just trying to eliminate some common stuff before I proceeded down that path. thanks so much for your replies. I am a pretty good auto mechanic, but there are definitely some intrinsic differences on this marine application I am not familiar with.

Actually the only real difference is the lack of vehicle speed sensor and oxygen sensor. Did you check fuel pressure at the rail at wot? did something simple like the vacuum hose come off the regulator?

I checked fuel pressure with key on, appx 40 psi then pump shuts off, then @ 1800 rpms, got appx 37-38 psi. have not checked fuel psi while running, will do that this weekend. did trace out regulator hose, it is hooked up. when I disconnect it I pick up 3-4 psi of fuel pressure which is what I understand should happen.

Make sure your oil is not over full. Yes the oil. Newer oil pans only hold 4 quarts or so. 5+ quarts and you will lose RPM.

Checked fuel pressure while running, 37-38 psi @ 4800 rpm, disconnected vacuum to regulator came up to appx 41-42 psi. Still having the same power loss issue, run 4800-4900 for 30-40 minutes, loses about 1000 rpm, put around for a little while, its fine. I can run 35 mph which is 3800 rpm for however long I wish, but then run her flat out for awhile, she just drops that 1000 rpm.