Mysterious car problem INFINITI G35 hiccup, skip, slippage? transmission?

I have an '03 Infiniti G35 sedan A/T. I don’t know what it is, but the best way to describe it is like a hiccup or skip in the engine or transmission. I don’t think it’s the engine because when it happens the RPM’s don’t go down. I’m pretty sure it’s the transmission or even the mounts. But it only happens randomly. I’ve already flushed out my transmission oil and it still hasn’t helped. Can anyone solve this mystery?