I have a Mercury 50 HP, 4 cylinder motor, 1980’s (with brown stripe). For 6 years now, it has run on 3 cylinders for the first 5 minutes then all 4 for the rest of the day. It seems like once it is warmed up it runs fine. I’ve switched parts and wires to rule out plugs, wires and coils. I am wondering if it is the trigger or switch box. The Serial Number is 6452452.