You dont mention what powers your fuel intake system/carb /or tbi/well given the age of the vehicle its the only two choices I can
bring to mind,Anyways sounds like you have an altitude
problem ie not enough gasoline;sounds like you have to rejet your carburettor or change the chip in your ECM to compensate for the rarified air,plug in a code scanner and see what the buggers doing in real time,while running ,this willgive you a more comprehensive picture of whats happening