Probably running lean at WOT. If it keeps up you'll get a CEL.
There's two possibilities.
1. Somehow you damaged the little wires in there. Doesn't sound likely since it runs okay at lower throttle.
2. It needs some "streamlining".
Try this: the flat "floor" under the "tunnel" has a fairly blunt leading edge after you cut the MAF. Take a very small file or some sandpaper and carefully thin that floor down and make the leading edge sharp.
This has helped on MAF mods, especially 4.0 mods, but my 3.0 benefitted from it also. I've posted on this, but never updated my mod page.
Most folks don't need it, but it sounds like you do.
Do you have the plastic or metal tube sensor? If you have the metal, this might not apply and then I can't help you -- I've never done a metal one. But clean up any rough areas that could be causing turbulence anyway.