This is happening to my '05 G35x as well!! The RPM's "bounce" (aka "surging") only on a cold engine when I'm accelerating between 30 - 60 mph. Usually this starts to happen when the RPM's hit ~2500 at around 35mph. Oddly enough, I do not have any check engine lights on my cluster. I used my laptop with Nissan Datascan II and sure enough, it has the P1815 code stored in the TCM (along with another code numbered P1532...?).

One thing I noticed about the car when it starts to surge: if you immediately switch the car to cruise control, the surging with quickly calm down and go away and the car will cruise without any issue. At first, I wondered if this implied that the throttle position sensor is going bad. I haven't ruled that out, but it sure "feels" like there's an electronic issue that is causing the transmission to hesitate in its shift from 2nd to 3rd gear. If anybody is still interested in this, let's get scientific on this bitch. I'm trying to figure this **** out. It's annoying, tbh. Makes it hard to drink my morning coffee on my way to work when the engine starts shaking as I merge onto I-75. I know this can be fixed! It's just a machine!!!!!!!

 

P1815 is the manumatic switch not functioning properly, this is pretty common and your manumatic mode probably doesn't work. Usually it's just the switches need to be cleaned out thoroughly with a plastic safe electrical contact cleaner.

P1532 though I have no idea, are you SURE that was the code? If so you need a different scanner that handles protocol ISO 9141-2 because it's misinterpreting the code.

 

Here’s my laptop screen using Datascan II. Shows P1815 and P1532

 

Wow that's weird that NDS is telling you that code since it should definitely have the correct protocol, lets break the code down

P - Powertrain
1 - Manufacturer specific, not an industry generic
5 - Speeed control / idle control
32 - Manufacturer specific error within the previous catagory (speed control / idle control)

Speed control and idle control for the context of the V35 chassis is mostly just to do with cruise control, brake switch, wheel speed sensor, etc.

That specific 32 code doesn't exist for Nissan/Infiniti through the V35 chassis years, but NDS is definitely telling you there's SOMETHING it's finding.

Later V36 chassis also added some battery VOLTAGE monitoring in those code ranges, have you been having issue with a dead battery?

Earlier Nissan chassis didn't even use the P15XX codes.

I think you should go down to a local parts house and rescan to see if they pull up the same codes.