[2ducs]

Most likely if you have the um161 chip installed you're running richer now than you would be if you reinstalled the stock chip. And being at altitude it will run leaner due to less O2 molecules. Hence why it runs better at lower altitude.
You could try trimming the pot to see what kind of results you obtain in your area; LH rich and RH lean. Be very careful to note when you have reached the limits as you can damage the pot if forced beyond its travel.
Good luck.

 

[2ducs]

If you know which chip you have then I thought you knew about the potentiometer in the ecu you can adjust for fuel flow. My bad. Have a look in the ecu with the rubber plug removed. You will see a square device next to the chip with a very small slot for a jewelers size screwdriver. Turn the screw ccw for rich and cw for lean. A small adjustment may help alot.

 

[2ducs]

less o2 would make for a rich condition? the carb guys let more air in as they go up, usually by opening the air box.

Just to clarify what I meant about the lean at higher altitude. I was referring to the fuel injection system being temperature and density compensating thereby less fuel at less density unlike a carb that is not conpensated for density and would be RICH at altitude.