Why is my car cutting back?!

The other day I noticed that when I floor it it starts to cut back around 5000 rpms in 1st and about 3800/4000 in the other gears. This is really pissing me off because i just dumped about $1500 in top end parts, and found out the other day my steering rack is bad ($510) Why is it doing this? My friend told me it might be my fuel pressure, I was thinking that my injectors might be shot since they're the stock ones and have never been replaced. What do yall think? Thanks!

I forgot to tell you the symptoms: It hits 5000 and you know what its like when a plug wire falls on your headers? Its like that just not as bad, it coughs, and sputters but if you let off it runs fine just not as hard as it should.