I have an 8.1 mercruiser in a 360 and it run smooth up to 4000 rpm and then starts to surge...42-4400 rpm back to3900 and back and forth. After removing the vent for the gas tank it ran for a while and then started pulsing again.... Then after removing the gas cap while at wot ,the engine ran perfectly......we ran for an hour at 3400 rpm without any problems....after that we ran the motors for 3 hrs to travel down the lake....again after trying to go to wot , the motor started to pulse and surge again.......I have twice changed fuel separators and put in new plugs....any suggestions ???????? We had the motor scanned,but no problem codes came up....