1988 Yamaha 60hp - sputtering high revs ?

HI - I have a 1988 Yamaha 60FETO (60HP) - having an issue with it dropping a cylinder at high revs - it can sit in 4000RPM all day no issues - but when pushed to 4200+ RPM - after approx 3 mins its drop a cylinder for about 1 min - then kicks back into life and runs normally for about 3 mins then drops out again ? - I have changed out the fuel pump / coils / cleaned all the carbs / changed the primer in the fuel line / cleaned fuel filter / new spark plugs - it has 120 psi on all 3 cylinders - when it happens pumping the primer does not do anything - any ideas what may be going on here ? vapor lock ? / it has no fuel leaks / fuel pump is sealed to the motor well - I am a bit stumped with what may be causing this now - hope someone can give me some ideas - all the best