When I start my 1996 225hp mercury offshore, it runs fine until about 1400rpms. When I try to go slowly past 1400 toward 2000 rpms it wants to stall. If I am lucky enough to get pass 2000 rpms it runs like a top all the way up to 5200 rpms. If I shoved the throttle all way forward from stand still, it will accelerate to 5200 rpms. When I ilde down in a no wake zone it stalls when I try to speed up again. Tried venting (remove gas cap), water seperator, new plugs, independant tank(same results), fuel additives (sea foam), with no success .