• temp gauge running ~90C on the road Normal, maybe a bit on the hot side
  

• come to a stop and the AC starts coming out the side dash vents (outside temps ~ 30F) Not AC, just outside air. Again, this can be normal at a stop/idle depending on the car.
• AS SOON as I accelerate and start rolling, the air coming out the dash turns warm/hot Normal given above circumstance
• temp gauge almost IMMEDIATELY falls down to ~ 87 - 90C You didn't say what it went up to...? Sounds like they may not have properly burped the system so there is still air in it. Also you could have a stuck thermostat but that is less likely being straight from MB. All of these symptoms point to either low coolant, air in the system or lack of circulation.

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth