still stumped

[twopunches]

i have a 1992 mercury 115.this is my problem.when my motor runs between 4 and 5000 rpms for a few minutes it looses all power and shuts down.sometimes it starts right back up and when it doesn't i have to wait for it to cool down before it will.also the alarm goes off.Ive had the water pump changed,new thermostat, new plugs,and wires.when out on the water Ive taken the cowling off and felt the motor.its not running hot.it runs fine until you run it between 4-5000 rpms. for a few minutes. it doesn't show up on a diagnostics either.ive also converted it over to premix.im sure i dont have any fuel issues.
any ideas? thank you in advance