AQ131A loses power

First a worg of gratitude for all the help in the past.
Last season the problem started and it didn't go away with a carburetor exchange or new plugs but it seemed to go away when I ran engine off of fresh gas from a jerry can. This season, I hand pumped all the gas out of the gas tank(used it in my car with no problem) and refilled the tank.
Engine has Pertronix electronic ignition (thanx MarineEngine Forum) and changed spark plugs last season.
Engine started right up and exhaust seemed slightly more than I rember last season but not heavy white or black. Warmed engine and then took out on lake. For 5 minutes it ran well and hit 30mph (previous max speed). Then it started happening again. The RPM just fell and I was able to keep the engine going by puuling back on the throttle and pushing gently forward. This rall in power repeated several times at times the engine cut out. Each time it restarted , occasionally requiring a partially open throttle to start. When I got back to the dock, I let the engine idle and several times it slowed to stop. With a slighlty higher idle speed and with the throttle open a little, there seemed to be occasional falls in RPM as well. No backfiring noted.
Any ideas??

Yup,The vent hose from the tank could be clogged.The pick-up tube has an antisiphom valve,it could be clogged or the tube itself is clogging.I have found beer tabs and plastic and cigarette butts etc.in the tanks.
Worst possible is the fuel pump is bad.Get a fuel pump pressure tester.Jerry

Jerry brings up some valid things to check. Here's something simple you can try (you may unknowingly tried already), go out, and when it starts acting up again, open up the engine hatch and see if that changes anything.

thanks.
will hopefully try looking at vent and antisiphon issues this weekend.
will post results