I have seen 80psi stactic pressures on my 94 940 with a high ambiant temp and after it has been setting in the sun. These systems do have a high pres cut out switch, on the front of the condenser. Can't remember which one/color it is. Since it does not have a factory high pressure valve, it is hard to tell if it is actually building excessive pressures or a bad switch. It sounds as if there may be an obstruction in the system, ie. clogged oriface tube or accumulator
. If it was me, I would discharge, deep vacuum then add the correct amount/weight of refrigerant and see what it does. If it still wants to be ignorant, then it is either the switch , clogged item or on to plan b. Plan b being pulling and flushing all components.