My car is a 2005 g35 sedan automatic RWD with 139,000 miles on it. As of late it has been doing some weird things. When in idle, for example, at a red light, it feels like it slips, or as if someone has hit me from the back. And now, when I try to accelerate, it does not have the power it use to and seems to start stuttering and at one point I thought my car was going to fail on me. When this was happening, the check engine light was flashing and then turned off. Is this a sign of me needing a new transmission. Oh, and by the way, when I change gears from park to drive, my car does a huge jump.

 

See if you can pull any codes.

 

What do you mean by "codes"?

 

Not sure what the official naming for it is. But essentially lets say you have a Check Engine Light. The computer throws a 'code', that can only be read by a scanner that's plugged in.
So what's being suggestion is if you have that OBD scanner, you can scan your car and see if it has any codes, despite there not being a light on the dash, there still might be a code.

 

Recently had a similar issue i had a small leak from my radiator, low coolant caused the car to go into limp mode. I had my buddy (Nissan Master Tech) clear code, and top off the coolant, i top it off 1 every 2 weeks and even then its a very minimal amount of coolant i add. I am currently looking for a new or used rad Ive priced them they are about 550 used i saw a used one from a salvage yard for 200.

 

Check your fluid and see if it's burnt or smells funny. One test I saw how to differentiate engine vs. transmission problem:

Rev the engine high at idle see if it acts up.
Try switching to manumatic and see if it acts the same way.

I bought a new replacement radiator for my coupe from eBay for 60 bucks, lol. Swapped it out myself and haven't had issues for almost 20K miles.

 

I had an issue where my transmission was stuck in 4th gear at a stop, and stayed that way. It threw like 4 codes, 2 of which were p1757 and p1759. Mine's has 150k on it. My trans oil pan was rusted out and the fluid appeared to be very old and dirty. I changed the filter, gasket, pan, and trans fuild (drain for a couple of days since it was leaking from a crack) and when i filled it back up it ran like a champ. Just letting you know, if your pan is leaking you can get one online for the 04 g35 sedan, which fits the '05 X for 50$. http://www.infinitipartsusa.com/2004...tm?searching=Y the filter kit and gasket can be found online too http://www.partsgeek.com/catalog/200...ilter_kit.html. I'm using autozone ATF that has the nissan matic J or S or whatever the hell it takes. I suggest using an aftermarket trans oil cooler too, I'm going to invest in one. My bullydog tuner (POS btw) reads my trans fluid a bit over 225f sometimes.

 

I had similar issue with my car as well. The problem on mine was not the transmission itself but the ECU/valve body. I got my car brand new and the first time my transmission failed was at 2.000 miles. The problem then was the valve body inside the transmission. Dealer at that time said they found a lot of debris in it, even though majority of the miles came from me driving cross country. The second time my transmission failed was about a year ago. Again, all the shops kept tellingm e its my tranny and i need to rebuild it and all that BS. I was blue in my face telling them lets look at the valve body but no one listened.
Long story short, you CAN NOT get another valve body or ecu because those can only be had from dealer as a complete unit and the cost is about $2,000. What i ended up doing was buying a junk yard transmission with less mileage on it and putting it in my car. When we took my old tranny apart, aside from dirty oil, there was nothing wrong with it and again the valve body was to blame.

My advise, have a shop pop of the tranny pan and see what the deal is. if you see a lot of grime and debris in it, then you may have a need for a new transmission. If the oil is dirty, but no major dirt, try and source out a new transmission ecu/valve body and swap that out. That takes about an hour and you do not need to drop the transmission. You can also have the shop to their best to take the solonoids apart and clean them out.
On the off chance you can find a junk yard tranny with less mileage, keep it on the side and still swap the ecu/vb and when tranny goes swap that and replace valve body.