'93 175 2 stroke engine hesitation

[JamesAnderson]

My 21ft Grady is powered by a '93 Yamaha 175 2 stroke saltwater series which periodically hesitates between 35-50k RPMs. Last summer I replaced the fuel filter, and burned through a fresh tank of gas but the periodic hesitation with 3-4 rpm variations persists. This spring I took her out and took her gradually up to 50k rpm. She faltered a bit at 37k rpm but ran smoothly at 50k (her current max) for a 15 minute trial run. It seems that the hesitation occurs less after a minute with the engine off.
Any suggestions?

[boats.net]

Run a vacumn test on fuel system to see if there is a restriction in fuel hose or tank pickup in boat. Check fuel pump diaphrams, remove and clean carbs.