200 kph is going pretty fast lol. Are you cruising at 110 kph when you get the KR? What is your OAR? Mileage on your Flex? Which motorcraft plugs? 534, 542, 578 or something else?
Generally see more success with 100% rather than 50/50 methanol, but a good tuner can work around that. Are you sure it is kicking in when pulling from 110? Is it on a progressive controller?
Any lesks? KR if you pull it from duty completely? What is your AAT, IAT, and IAT2.
I think I have seen more Flex's report cracked/windowed block than any other transverse application. A compression test might be in the cards for you. If you have a usb borescope you can check the intake valves, cylinder walls, and piston tops. Not sure about rings, which usually is what give way, unless you have LSPI.
If you have datalogs, we
have members and tuners here that can use them as a disgnostic tool.