Mercury 115 ELPTO stutter at operating temperature

[bwguardian]

I am trying to chase down what I think is a fuel problem. Motor is a 1999 115 2 stroke...fresh rebuild...with about a dozen hours on it. The last tank I topped off...1/2 full...with regular gas to finish running double oil for break in. The engine starts and runs fine. Once up to operating temperature it starts to randomly stutter rpms...not much, but noticeable. This is particularly around 3,500-4,000 rpms and when it happens, I can throttle up to 4,500-5,000 rps and it goes away for a bit after returning to lower cruise speed, then returns. When leaving the boat ramp it's okay, then 15-20 minutes into the trip I start slightly noticing. It's when returning back to the ramp after fishing it gets bad. Compression tested and fine, all new fuel lines with water separator filter, new primer bulb (2nd one as first suspected), no anti-siphon valve, and able to blow through lines to know there is no blockage. When it gets bad I check primer bulb and it's not flat but can squeeze and rpms will pick up a little. The only thing I haven't replaced is the fuel pump diaphragm but I took it apart and tested...disconnected fuel supply line at top carburetor and was pumping fuel. I am not convinced this isn't my issue but would like to isolate the problem to boat or motor.