Limp mode
 

Ok so I put the K&M cold air induction and a Gemini cat back on the car. Ran great for awhile then the check engine light came on so I brought it in to the local Nissan dealer and they cleared all the codes. Get it back runs like a champ! Pulls hard, etc. about a 100 miles later it goes back into limp mode. Hardly able to do highway speed. You can floor it and it seems like the TB's are only opening 1/4 of the way. So I get the my destination was only in for about 15 minutes come back out thinking it was still in limp mode and runs like a champ again! Accepts full throttle, pulls hard, etc..... WTF! When it was in at the dealer they side the code was indicating "left bank"? Any help before making another appointment at the dealer?

Buy/rent/borrow an OBD tool and see what codes are present. Post codes in this thread.

Edit: Many auto parts stores will take their OBD tool out to the parking lot and read your code at no charge.

But will a code be present after it's back running good? Are they stored in the ECU?

Many codes will stay in the ECM until cleared using an OBD tool (or an ECM reset is performed).

I'm getting really F'ing sick of this car! Now the check engine lights on (as to be expected) so I'm setting up yet another appointment with the dealership! Really getting irritated with this thing!

Do a little investigating on your own and you could save yourself a bunch of money. Get an OBD tool and see what DTCs are present. Post the DTCs here and someone will probably be able to tell you what is wrong and what it will take to fix it. Might be as simple as cleaning the MAFs (15 minute job even if you've never worked on a car before).

Ummm.... Don't blame the car for issues with aftermarket parts... Sometimes this happens.

With that said, scan the car and tell us the code. It could be as simple as loose gas cap. Let's get to the bottom of this and "enjoy the ride" :)