Throttle problems with Suzuki DT 140?

I have a 1988 Suzuki DT 140 that I am having a problem with. The engine starts and idles fine. When I drive the boat at full throttle, it will run fine for a little while and then bogs down to a much lower speed (about half). It bogs down at different times when giving it gas, sometimes while accelerating to full speed and other times a couple minutes after being at full speed. It also bogs down after going over waves. About the engine: I bought it in March and replaced most of the gaskets. I replaced the gas but not the oil (I don't know if this could be a problem). I replaced the throttle, spark plugs, and impeller. When I ran the engine (drove it around for about 1 1/2 hours) the oil level did reduce, so I know it is mixing. I can only assume that the engine sat for a while before I bought it. Any help would be greatly appreciated.