my 2006 fusion 700 was a little hard to start today and it stalled a few times..i finally got it going ,stopped for some gas, and then rode a good few miles..it started back firing a little bit then basically lost power..still starts but no power to move and it back fires continuously and sounds like a vw (volkswagon).. please help im really nt sure where to start..tried putting in a diffrent set of plugs but no help..what do i try next