[s-cars] S4 hesitates , stumbles at elevation

[George Verbryck]

Well my 93 S4 has been running great and I recently had a chance to take it up to the mountains for an Oktoberfest event. During my drive up every time I tried to pass i.e. WOT the car it would stumble , retard timing or something it is hard to explain other then full power was not available. Once down I gave it a few spirited WOT runs on the freeway and it responded by charging out to the redline no problem. I looked  for codes by blink method and got nothing . Any ideas where I should start looking for the problem or what the problem may be? I thought the altitude compensation on this car and motronic was computer controlled? TIA , Regards, George