Welcome.
Let me start by saying that I don't like the Merc powered boats, so take this with a grain of salt.
The Merc powered boats are great, as long as they don't have issues. SeaDoo no longer supports them, and some Merc dealers wont work on them either. The parts are very expensive, and now there have been reports of some of the engine sensors being discontinued. Also, since SeaDoo only used that drive system for a few years, we don't have too much knowledge on them here on this board.
With that said... I would not buy that boat, unless it was VERY cheap. You could find yourself quickly dropping $500 to $1000 on it for a couple fuel pumps, and since it's an intermittent problem... it may be hard to track down without the help of a dealers computer to plug into the fuel system. (assuming it's fuel injected)
If you do buy the boat, we will help as much as possible, but the fuel system is complex. There is 3 fuel pumps, and a goofy fuel mixing tank, and a recirculating injection system. My guess if it runs at full throttle, but stalls at an idle, the regulator is bad, an injector is hanging open, or the TPS sensor has gone bad.
Good luck, and let us know what you decide.