Having some motor trouble

I have a 1994 30hp johnson motor on a small pontoon boat.

The motor cranks right up, idle is fine. When you first start out it will go wide open with no problem and runs good.
After ideling around or going slow it wont go past 1/4 or so throttle without acting like the plugs are fouled out.
If you let it sit for 30 min or so it will run fine again.
i noticed a very small amount of oil at the intake of the carberater.
I have changed the spark plugs.
new fuel.
openened the valve on the gas tank for air.
any help would be great thanks

You may want to look into the impeller / cooling system.-----Overheat can trigger a warning to prevent full throttle operation.----------You can use a timing light to see if ignition problems show up when you try to open it up.------Compression test is also advised.

Thanks, It has to be something simple and small, it just runs too good at first...