We have had several occasions where our '86 Westy suddenly began running 
VERY rough - as though it was going to stop - and after scaring us to 
death would be OK after stopping and going again. 
 
Let me set the stage a little - we live at near sea level in the San 
Francisco Bay Area and make trips to places like Albuquerque, Grand 
Canyon, Las Vegas and such which are all at somewhere above 5000'. 
 
In each instance, we have been traveling along just fine and all of a 
sudden, the engine will get very rough and loose power. For some reason, 
this always seems to happen when we are :
1. At altitudes above 5000'
2. In the middle of nowhere.
3. When it is either cold and raining or 120 in the shade.
 
Each time the problem either went away after some 30+ minutes of limping 
along, or after we stopped in some town, and was gone when we started the 
engine again.
 
Nothing seems to be predictable and my mechanics, having nothing to work 
on but my harrowing tales, haven't a clue. 
 
The last time this happened, we were going south out of Las Vegas, had 
passed Boulder dam, and were on a two lane road through the desert. We 
were about 40 miles south of "Resume Speed" and a good 100 miles from the 
nearest town. Every town marked on the map turned out to be a place where 
it said "Welcome To <wherever>" on both sides of the same sign and the 
population numbers were on these little tags that hang by nails in the 
sign... and there were only two nails!! Needless to say, we thought we 
were about to be broken down where no one could/would help.  I had to 
shift down to 2nd just to keep the engine at high enough RPM to keep it 
from stalling. 
We managed to limp into the next city (had a gas station of some 
nationally known brand) and stopped. I really thought that when I turned 
off the engine, that would be the end of it, but after getting some gas, 
it fired right back as though nothing had happened.
 
Anyone have any ideas of what this was all about? It has happened more 
than once and not only in the desert.

/Jack