Just recently my 08 G37 coupe (140,000 mi) started randomly having an issue where when the pedal is depressed by any amount, even all the way down, it will only accelerate as much as barely touching the pedal would. But then seemingly randomly it will return to working fine.

There's no error code showing up yet, but a speed sensor error was present a while ago. I've experienced limp mode before where it caps rpm at 3.5k, but this is different. I'm wondering if this is the car limiting itself or the gas pedal malfunctioning? Thanks for any and all answers.

 

Speed sensor error on the transmission usually always points to a defective TCM located in the valve body. If you have had this code in the past this is most likely the fault again but it doesnt always trigger. This same code could be putting you in limp mode since the trans can't detect the output shaft speed correctly it will hold the engine from proper acceleration.
Try to keep driving in that range to get A code to show up and work from there. If the same speed sensor code pops up then start looking at your TCM.