Good points yager, but remember, both Haynesworth and Rogers are guys who had the same problems with consistency that you just described for Ngata. Both Haynesworth and Rogers have really just come into their own within the past two seasons after years of flashing such potential.
In my opinion, Ngata is very well on the same path as Haynesworth and Rogers were in becoming a consistently dominant NT. Remember one more thing though, this is Ngata's first season as NT in the NFL (Gregg used to be the NT). Haloti is playing A LOT of snaps. I think he is being overworked to an extent, which could contribute to his inconsistently.
Haloti has gotten better each year and is well on his way to becoming an All-Pro DT. Right now the AFC just happens to have the best NTs in the league with Kris Jenkins, Hayneworth, and Rogers. If Haloti played in the NFC, he would be a Pro Bowler. Kevin Williams is the only dominant DT in the conferance.
Haloti's time is coming, and he has certainly been an impact player this season. He will only get better and projects very well to a Shaun Rogers-type of player.