2008 G35s, manual, rwd, 95k miles. I just replaced a tire due to blow-out and thought maybe the new one wasn't balanced properly but I had that redone and it didn't seem to be it. The car vibrates like mad and lurches along (as if the wheels were square rather than round), but only after I've been driving for 10 or 15 minutes, no matter where, road condition, speed, etc. It feels and sounds like maybe something is loose in the front end (all the time, not just after 15 minutes) but I have no idea what or whether it's related. What could possibly be happening only after 15 minutes? Once it starts, it continues until I shut the car off and start it up again later. Thanks all.

Very good chance it's the OEM dual mass flywheel springs starting to fail, they will cause random "rumbling" feeling when the springs become heat soaked and become very loose after driving. It will also have symptoms feeling like the engine doesn't have as much torque when you take off from a stop.

Take it in for a diagnostic at Nissan make sure to tell them the symptoms are only present when the engine is hot after driving, schedule a time when you can literally drive right into a bay without turning off the engine so the tech can observe without spending time trying to "reproduce the issue".