High head pressure can be a couple different things, if the ambient air is very hot it can cause high head pressures. It could also be low water flow. But by the way it sounds is it is tripping off on high head pressure because of a combination of things. The hot ambient air and the unit being on the end of the loop not getting the correct flow is more then likely causing the problem. When you say the water temp is 89/98 is that the load side water or the source side of the unit?? I would need more information to help you with what's going on. I would start with taking source side delta t and the pressure drop on the units when they are running on a hot day.