1985 9.9 4 stroke stalls and hard to crank

[clydetwo]

3 proffessional mechanics have attempted repairs. Most recently new carb and water pump impeller installed. no improvement. Motor very hard to crank. Once cranked runs smooth for various periods of time. Anywhere from less than a minute to 10 or 15 minutes. Motor runs smooth, then rough and then stalls. 50% of time motor stalls when increasing or decreasing speed. Any recommendations?

[boats.net]

May be fuel pump, try pumping primer bulb on fuel hose to see if it helps it run better.