140 Merc losing power

I have a 140 L6 Merc, I think either a 1980 or 81 (SN 5742061). Unit starts and idles great and when I first take it out it will run at full throttle great for about the first 20 minutes of travel....after that it acts like it's running out of gas. It will lose all power and I can take the throttle back to idle position and it will stay running at idle speed, but when I try to get back on plane at full throttle it pulls hard for about 10-15 seconds and then loses power again like it's running out of gas. I can engage the choke when it's losing power and this doesn't help. The fuel line squeeze bulb remains relatively firm so I don't think it's losing flow of fuel.....squeezing the bulb doesn't help remedy the situation. I've taken it to a local Mercury shop and they said that the stator and rectifier needs to be replaced ($500!) but when I do the diagnostic test as outlined in my Seloc motor manual they both check good. I'm at a complete and total loss as to what to try or even check next and I'm not enthused about changing parts for the sake of changing them to see if it fixes the problem. Any ideas as to what could be causing this issue?

Try running on a different 5 gallon tank.--------------------Have you verified that fuel pump is working properly.------Water pump impeller must be maintained ( replaced ) every 2 years as well.--------You can test with a timing light hooked up as well. What happens to spark when motor falters.

racerone said:

Try running on a different 5 gallon tank.--------------------Have you verified that fuel pump is working properly.------Water pump impeller must be maintained ( replaced ) every 2 years as well.--------You can test with a timing light hooked up as well. What happens to spark when motor falters.

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It has new fuel pump diaphrams so I assume the fuel pumps are working properly. The motor is now on a different boat and it exibits the same bahavior on both boats so I don't think it's a fuel pick-up from the tank issue. Water pump was installed new last winter. Not sure what happens to the spark with it loses power....I don't have a timing light and I don't have a means to check timing while the motor is running full throttle across the lake.