1996 SEADOO XP Runs Great Momentarily

[Ponykeg02]

I bought my 1996 SEA DOO HX last summer from a guy who had just had the motor rebuilt. I believe it has a chatter box or some modification to the wiring harness.nothing custom tho. ignition is gone and just has a start/stop button. my ? is when i hook a garden hose up to it and run it it runs great for as long as i want.when i put it in the water it runs great gor a few minutes then just dies. feels like electronic problem but also thought maybe its overheating.starter wont crank the motor over at that point but trys to.also is set up for pre mix. any suggestions?not too familiar with this ski yet. waterpump maybe?