I have a 2013 Infiniti M37 whose XM radio suddenly started to have issues. I can start the car and it works fine for about 3-8 minutes, then it starts to break up and then says No Signal. I have searched forums and other resources but no real conclusion on what the issue is. With it working fine for a few minutes (whether standing still or moving), it almost seems to be something like once it heats up it cuts out....I just don't know. Infiniti says it will be around $200 to diagnose, $750 to fix if it is a radio issue, and around $3500 if replacement is needed. Just wondering if anyone has run across this issue and could lend some insight. Seems replacing the antenna might be a feasible troubleshooting step (and probably the least cost), but I would think if it was an antenna issue it would not work at all (or be sporadic all the time).

I welcome all insight, Thanks!

No, it's almost certainly the antenna. If it was open wiring or high resistance, then it would be misbehaving erratically or all the time. The antennas aren't "dumb" hardware, there's plenty of silicon in there. What you're describing is pretty common when they start to fail.