2000 mustang stalling out around 4500 rpms

[Steeevooo]

Ok so im beyond puzzled about what to do with this thing. Its a 2000 mustang gt with the 4.6 in it, we've had it on the diagnostic machine and it told us to change the number 6 coil pack so we did that and all the spark plugs while we were at it. Bought a new alternator as well and fuel filter...The problem stopped for about a day and sure enough we got on it and around the 4500 mark it hesitated terribly to the point where it caught itself back around 3600 rpms. Now if we slowly jump into the rpms it doesnt do it but if we floor it is when it bogs out. The only other thing I can think of is the fuel pump but I wanted some opinions before we go and do that nightmare job.