I had a similar condition where absolutely everything in the PP checked out fine but after 2 years of use it would only hold speed for a little while and then would drop the setting.
I contacted the techs at PP and gave them a good description of the problem and they sent me back an easy fix.
It turned out that my throttle cable was to blame. After a few hundred hours, the throttle cable slides with less resistance and the return spring will pull the lever back to point where PP cannot make up the difference.
Solution is what is called a cable brake which is basically just a cable clamp about the size of your throttle cable, placed close to the control side. You can buy them, but I went to Ace Hardware and just bought two cable clamps. You then just tighten them down until the control lever has a little more resistance.
Now the PP is dead on and won't let go even in the worst water.
Big ups to the PP people for the $1.10 solution.