GT40 mystery problem -- Advice needed.

[aeh]

September 13th, 2023 -- Boat: 1997 Ski Nautique 196 with GT40 engine
Description of Problem: Engine fires up immediately, accelerates like a rocket when pulling up a slalom skier, and runs like a champ. However, after approximately 15 minutes of slalom skiing at 32-33 mph, the engine cuts back abruptly but does not turn off. In 3-4 minutes with bilge blower on and engine at idle speed, it will accelerate strongly again and allow 5-6 minutes of additional slalom skiing before symptoms return. Temp gauge reads exactly where it should.
New low pressure fuel pump, new oil pressure switch, new/correct 160-degree thermostat, and fresh 15W40 Rotella oil and new Motorcraft FL1A oil filter had no effect.
New impeller. Water intake screen checked and is clear of any accumulated debris. Fuel: 90-octane Non-Ethanol.
Problem seems heat-related in some way.
Today, as an experiment, between slalom runs, we would turn off the engine and prop open the motor cover for several minutes before the next slalom run. The problem did NOT occur today using that procedure.
What is the underlying problem? I will be very grateful for advice!

[malibud]

replace the fuel relays cheap and easy

[aeh]

Thanks very much, Malibud, for responding to my post. I will proceed in following your recommendation.

[Peter6000]

First thing to find out is if its fuel or spark related, measure fuel pressure on the rail when the problem occurs should be above 30 psi.
For spark you can use a timing light and check advance while driving just be careful.
The TFI module likes to fail when hot get a can of freezer spray and cool it down see if the problem goes away.

For temperature the engine has 3 coolant sensors one is actually a switch.
1st for the gauge
2nd is connect to ecu I would recommend to test it.
3rd is a switch that turns on (pulls to GND) when engine goes over 190 deg this will turn on the check engine light and put the engine in limp mode
--limp mode will bring idle up to 850rpm and lower max RPM to 2500
If you feel you are in limp mode you can temporary disconnect the oil pressure switch and coolant high temp switch.

If you don't have it already download the GT40 service manual
https://www.google.com/url?sa=t&rct=...V&opi=89978449
It will show you locations and test methods for all sensors.

[aeh]

Thank you, Peter6000, for this very substantive and helpful information. I appreciate your taking time to respond to my request for advice.