2007 115 4 Stroke Mercury

[scottemma1]

I have a 115 Mercury 4 stroke that idles badly. It supposedly ran well when tested at the Marina where i purchased it, now won't run after 5 hour trip home. It will start on high idle but an alarm is beeping until i bring the rpm's lower. When shifting into the normal idle position it idles for a short time and kills. at medium idle speed it idles up and down, up and down. i have changed the fuel line and tried a new tank. All gas was fresh. I would like any suggestion to what else to look for what might be causing the problem.