I bought my van this april and did some basic maintenance( replace air intake, new plugs, new air filter, new fuel filter, oil change, etc.) then drove it across the country 2000 miles. I noticed after about 500 miles that my check engine light would come on when climbing hills and as I drove more Ifound it to be slowly becoming underpowered little by little. As I suspected it was fouling plugs. After about 1800 miles of driving I ended up at the ocean and the check engine light stopped coming on no matter how steep the hill was when at altitude even slight hills would cause it to come on. It also started running stonger. After a few weeks I drove back to altitude, and sure enough at about 2000 ft. the check engine light started coming on again and it started running rougher when climbing hills. I suspect that it's not adjusting the fuel mixture properly to altitude, but I have no idea how I would go about remedying that and would love any advice people have to offer.

Thanks,
Dusty