power pack?

Hi, i have been having trouble with my 1995 115 2 stroke. It first started running rough at low rpm. thought i would take it to a yamaha service shop for a good tune up. they spent 7 hrs working on the motor, spark plugs, carb cleaning and a new head gasket and some other tune up basics. When the motor was running at the shop it was running good. When I took it out on the water it ran good until i throttled down and the same problem happend.

I returned to the service shop and they worked on the motor for several hours rechecked carbs ect. they could not find anything wrong, of coarse it ran great at the shop.

again returned to the water and it ran good for one outing. The next time out it ran very pourly. rough idle and ran at half power at higher throttle. It would for a split second rev higher/power up, did that several times without staying at higher power. Just runs rough now at all rpms no power. Ideas?

[boats.net]

Can you get the dealer to troubleshoot it on the water, if it's loosing fire on a cylinder then it will show up on the water.