I live at 7000 MSL and both stick and automatic Xfires, Mercedes and Chevy run fine. Checking for leaks has been suggested. At lower altitudes you may not notice it because the computer is able to compensate for the difference faster.

At 700 feet on my old carb cars, I have to install different jets and balance the twin carbs precisely but on electronic fuel injection it does not seem to matter. I also have to play with the timing a little on the carb cars. But that may be caused by distibutors that are 40 years old. There is some wear and tear even with rebuilding them. I also have some leaks from the throttle shafts on the older cars. I can tell this when I spray carb cleaner on the carb body and shaft. It changes the idle. Just my experience.