Jerk / Bang when 5AT shifts from 2nd to 3rd gear
 

I have a 2003 G-Sedan with about 150K miles on it. I never had the transmission serviced as my car has driven like a dream until recently. For the first 5-7 miles the car will shift very abruptly from 2nd to 3rd gear. The shift is so rough it feels like someone had rear ended the car. After the first 5-7 miles from a cold start it'll shift smoothly like it never had a problem. I've been searching high and low for a solution and I'm coming up with NADA! Any help would be greatly greatly appreciated.

The hard 2-3 shift on the first gen sedans is very common and nothing to worry about. I'm more concerned about the comment you've never had the transmission serviced, have you never changed out any fluid by either a flush or a drain and fill? Or do you just mean the trans has never been worked on?

Both. Never had it worked I or did a drain and fill. The car was given to me recently

Is the check engine light on or the AT light? Check the transmission fluid it's located at the back on the top engine will need a flashlight and a 10 mm socket with a 6 inch extension undo the bolt check the fluid when the vehicle is running see if its burnt or at the proper mark( in between the add/ full mark )

Bt all means do a couple of drain and fills on the tranny. Don't have it flushed as this can actually damage the tranny at this point. Why you ask? At your mileage there is more than likely a lot of wear and tear on the transmission and it's entirely possible that the metal shavings are actually filling the gaps between worn pieces. A flush will remove these causing slippage and a drain and fill will not.