I agree with the above mentioned items, but just check your lines too. One time I had a problem just like this and it drove me nuts. After checking filters and pick ups and carb, come to find out, there was a very small section of my fuel line that got exposed to some oil, so the hose was really soft, so under WOT that section of the hose would collapse and was cutting off the fuel. I felt pretty stupid when I found that little problem!!
