The air valve (top set of butterflies) on the secondaries could be out of adjustment, very tricky to set right if they've been messed with.
If they open to quick you'll get a bog when you go WOT if they open to slowly you'll get poor performance @ WOT.
The way you desribed your problem sounded like they may be opening to slowly.
Make a small adjustment on the spring (looser) and test drive it and see if it's better or worse and then make appropiate adjustments there on.
Hope this helps, Cp