Not certin I understand your problem. This 91 stops blowing cold only when
you get on the highway? Does it stop blowing cold when operating after a
short while, in any driving situation? I'll guess that you are a bit low on
refrigerant.What refrigerant you have, is being sub cooled in the condenser
when all the air from highway travel is blowing on the grill.Moisture is in
this refrigerant, and it is collecting as ice at the expansion valve,thus
plugging it up.When the system stops working , the ice will warm up and
melt. Then the system starts working  all over again(till the moisture
collects as ice again).I think moisture can be tested for.Evacuation for a
few hours , a new dryer,and recharging . is probably the ticket.Being on
the highway  is getting enough heat removed from the refrigerant ,to get it
cool enough, so that  when it makes contact with the expansion valve, it
will now freeze. A few degrees higher , and this wont happen.You can drive
slower or add refrigerant.Run the system for a bit , and then check the
tempature of the hoses(use your hands to tell)at the reciever. If the
tempature at the exiting( between the dryer and evaperater)hose is much
cooler than the entering hose ( the one that is hard to get to), you are
undercharged.I guess you can just recharge, but you will still have the
moisture in the system. This will get the temp up at the evaperater inlet,
enough so ice wont form.So how did the moisture get inside? My guess is a
leak in the low side, or from the person who serviced your system last
time.1/20 of 1 drop of water will shut some systems down. Try tightening
each connection between the compressor suction port (suc as labeled on the
rear of the compressor)and expansion valve. Just a bit.Best to evacuate and
recharge to get rid of the water.