Murcruiser trouble

[old mate]

My brother has been having a fair bit of trouble with his Bay liner, It has a Murcruiser in it and has been "fixed" by several different places. It last got fixed 3 months ago and has sat in the driveway since. today I decided to take it out, worked Awesome for about 20 minutes until I stopped. I went to take off again and it was very jolty and sluggish. I then got it up on the plane and set the throttle were it started to go sluggish and then come sweet and take off and then go sluggish again so on so forth.

Is this the fuel filter? Jets? Carby?

Any help would be great as it is starting to cost more to fix then what he got it for.

Cheers Chris