Could be the elevation changing the fuel mixture. How much different was the change? I no I have to change things on my snowmobile when going from wi to mountains in colorado. Or even driving to ca last year my car had no power in the mountains.
"If" everything is working like it's supposed to be then barometric pressure is measured at start up. The EEC should be adjusting for differences in elevation each and every time that the ignition switch is turned to "on".
There are a group of things that can cause the symptoms given by the OP.
1 or more o2 sensors out of range
MAF sensor dirty/bad
Baro/MAP sensor out of range
TPS sensor malfunction
The change in elevation by itself, shouldn't be causing the symptoms given by the OP. Normally I would say, "Just pull codes" but it seems like this has been going on for a while. By giving the EEC a reset then allowing it time to "re-learn", you should be able to get an idea of where to look by seeing what the EEC has to say about driving around locally.
It's very possible that for whatever reason, the EEC is going into 'limp' mode. It can be caused by a number of things as well.
It may be worth investing into a hand-held code reader or at minimum, finding a local shop or store that will pull the codes for free. Many parts stores will do this for you with the idea that they're going to get parts sales as a result.
On another note: Midget... When was the last time the car had a good tune up?
