I have a 98 502 mag efi that is exhibiting intermittent reduced power between 80 percent to WOT settings. Engine has about 500 hrs on, serviced regularly by same reputable shop and is Operated mostly in fresh water at lake havasu. When the engine starts to lose power it happens almost always when the air temps are above 95 degrees and after the engine has been run in these temps for at least 30 minutes. I think the common clue is higher air temps. When it happens it feels like it is not getting enough fuel. It generally happens when cruising and the throttle settings are pushed to at least 80 percent to WOT. Normal WOT throttle for my setup is 4900 to 5100. When down on power WOT RPM will drop to 4000 to 4400 RPM. The engine does not seem to miss, surge or run rough when this happens. The power loss feels like the engine just lost about 100 hp at the top end. The engine idles good and runs well at lower rpm and cruise rpm settings, and can be felt only 80 percent to WOT settings. Again the problem is intermittent.
Both the inline and water separating fuel filters have been replaced. The plugs and plug wires have about 150 hrs on them since replacement. Bilge area gets god air flow both in and out. Spark arrestor is clean. Fuel pump is original. Fuel tank venting has been checked and is good. Problem happens with 87 or 91 octane gas, gas from many different stations, when the fuel tank is full or near empty. A compression and leakdown test on engine normal for hrs on engine. Both the seawater and engine circulation pump have been replaced, water flow checked and good, boat has been lake tested by local reputable shop with laptop diagnostics, no codes or problems found during 20 minute lake test.