I have a new DF250 on a White Shark 228, at higher revs the DF suffers from fuel starvation, cut back to 3000rpm no problems, with a full tank it will pull 4500/5000 no problems, 1/4 tank 3500/4000.

Can anyone help?

 

Since your bike is new, it boggles my mind a little bit. It sounds like some sort of blockage in the fuel supply. Possibly inline filter or fuel petcock.

Also, since your bike is new. . . you could just call the dealership and ask if they know of existing problems in that model.