06 F115 won't plane up

2072 alum. CC, dual tanks. Steady issue for 2 yrs. now and my local marine dealer can't figure it out. Cranks and idles fine, about 3 out of 4 tries to plane up it will start sputtering just before planing up and will not smooth back out unless returned to near-idle speed. If it gets on plane, it will run like a spotted ape all day. We have tried plugs, filter at rear of block, VST filter, fuel lines. Diagnostics were run on the water with it sputtering and no codes. I can sometimes cheat the boat on plane by having everyone sit on the front deck to make it plane quickly before it falls on its face. Frustrating! Thoughts on what to look at next?