1999 50 stroke slows down

[bushmonk]

This is wierd ,1999 50 hp 2 stroke idles fine, throttles up to 5250.rpm for about 4 min. then slows to 3510 and stays there,pull throttle back to idle and then full throttle again and it climbs quickly to 5250 them after about 3-4-5 min slows down on its own again to 3510 has done this 6 times in a row,no alarms, lots of water from tell tale,squezzed fuel bulb but no change in speed.
watched carbs but nothing moves! no ignition miss it smoothly slows 3500 rpm and stays there ,this motor has the timing advance on the power pack run by a link rod and it doesnt move at full throttle when this happens either. can it be a defective pack? yamaha tells me $688.00 ***.

Thanks bushmonk

[99yam40]

Use a timing light to see if the timing is changing when problem occurs.
Timing light will also show a loss of spark to the different plug wires by moving the light to the different plug wires.
Use a fuel pressure/vacuum gauge to see if there is a problem with the fuel delivery, lines,pump, or maybe sucking air.
Check fuel pump to see if diaphragm has a leak/hole in it