2001 50 hp Bigfoot rolls back at WOT or under heavy load high RPM

[Det1]

I have a 2001 Pontoon with a 50hp 4-stroke Mercury Bigfoot Outboard. It starts right up every time and idles fine. I just purchased it this year so I don't know a lot of history, but the prior owner said he had one carb rebuilt last year and it ran fine. When I took it out the first time it would get to about 3200 rpm and it would start to roll back in rpms, if I feathered the throttle I could get it up to almost 5000 rpm but it would die at WOT. I verified with the prior mechanic that he had worked on it, but he had never lake tested it. I took it to a respected dealer and they worked on it for about 4 weeks with multiple trips to the lake and a couple thousand dollars in diagnostics, all carbs were rebuilt multiple times, timing sensors, replaced, and although it runs better, it still won't run at WOT. If I power up slowly I can get about 5000 rpm before it starts to spool back. The mechanic decided the problem was the ECM, so we replaced the ECM and that still didn't fix it. I'm at a loss, (of about 3 grand in work) and no idea why it has problems only at the top end. We tried pulling a tube behind it this past weekend and the extra drag made the problem worse and I had a hard time getting above 4300 rpm, runs fine up to that point, but then tries to roll back.