HELP ! OX66 250HP cuts out at 3600 RPM

[Permission]

Twin 250hp OX66 year 2000. Port motor will cut out at 3600 RPM. Cuts out completely as if the key was turned off. Starts immediately afterwards with key switch. Motor powers up perfectly and will run below 3600 no problem. a minute or two above 3600 and - off.
Have done ALL fuel related checks. someone suggested possible voltage drop via ignition switch ? or battery switch ??
Any advice is gladly received.
Thanks in advance.

 

[ClassicAQ]

If the cutoff were fuel related it would not be abrupt as you describe. A loss or restriction of fuel yields a steady loss of power. So I'd look to troubleshooting the electrical system, starting with checking connections at battery and engine. If good, volt drop test.

 

[99yam40]

have you monitored fuel pressures as the problem shows up.
if the VST runs out of fuel it will shut down like you turned off the key