I have a 1985 60hp yamaha which starts and runs well but after 20 mins. running slows to quarter speed for about 15mins. before running back up to full power. I have tried changing fuel tanks and have replaced the fuel line. Could it be the ignition system? and can the ignition system be repaired or repaced an what sort of cost is envolved. I have the engine on a Streaker ski boat near Toulon in the Med. Any advise would be much apreicated.