I would expect it to be an antenna issue - since it's working some of the time. The MX GPS antenna, from what I can tell, is located under the upper drivers-side glass of the rear trunk. Do you know what type of tint that was applied to your windows? Some tints can block cellphone and GPS signals - but I would not think that tint would have been applied to the glass on the upper section of the trunk as it is hidden behind the trunk trim.It’s intermittent. It will correct itself randomly. Or, it start off correct when I’m leaving my house.
I would guess that during the tint installation that the inside trim pieces, that surround the trunk glass, were removed, and that the GPS antenna, which looks like a small square puck, may have been dislodged such that it no longer is positioned correctly?
I expect that the Tesla navigation system can still determine the vehicles position if the GPS signal is lost (similar to when driving through a tunnel or in an underground garage), but the position will "drift" (become less accurate) the longer the system is without a GPS signal, and probably even quicker if the vehicle is changing directions (turning). This is probably what you are noticing with your system - it starts with an accurate GPS position, but as you drive, it displayed position gets further away (drift) for the actual position.
Great to see that you finally received your MX!