'06 F250TXR no throttle response past 4200 rpm.

[Resfox26]

Twin F250 on 33 Grady White. Port Engine runs smooth but will not accelerate past 4200 RPM's. Originally would run 4800 rpms cruise and loose power to 4000 to surge back to 4800. Now won't go past 4200 and no longer surging in either direction. Starboard WOT is 5800 RPM's.

Gas was pumped out of lines / inspected and fine. All filters including VST replaced but looked new. Sent injectors for cleaning and turned out to be in great shape. Yamaha Tech tested fuel pump and she's pulling fine as well as compression test (fine). Tech Pulled plugs (visually looked good) and removed 1 by 1 while running at 2000 RPMs. Drop in RPM's was acceptable in each cylinder. Looks like we're getting enough spark also. Going to test timing, and play around with the coils this weekend but wondering if we're missing anything or anyone has experience similar problems. I was going to check throttle linkage but I'm sure I will find no issues. Any help would be appreciated.

[Resfox26]

Update:

This weekend I pulled each plug and found a small crack in one of them. After replacing she runs like new. Lesson's learned....always check the cheap items first and a second visual inspection never hurts.

[rodbolt17]

pulling plug wires on an unloaded EFI engine is a teat on a boar hog and can severly damage the ign system on an F250.
if an F250 sparks at all it starts at 32KV.
depending on whats on my service order I would have started with a leakdown test then tested fuel rail pressure.
no good way to test spark Kv on the F250.
however while doing the leakdown test the cracked plug would look very different than the rest.
sometimes the best tool a tech has sits between their ears.
sometimes its the last one out of the box.