Merc 1150 issue

[wildturk]

Hello i recently bought a boat that has the Merc 1150 thunderbolt on it. Issue i am having is it runs awesome for about 10 min then when going at WOT it is surging from 4500 to 2000 and back up then just looses all power other then a trolling speed then i can hit the power again and it will get up and just sit right back down i have replaced all fuel lines/filter new gas tank. The engine did sit for a year. I am at a stop loss here.