UPDATE:
It is pretty obvious high attic temperatures would have an effect on pressure of the fire sprinkler pipes.
Likely the system has been cycling for the past 20 years high and low pressure depending on those attic temperatures.
I installed a thermostat controlled attic fan today and placed a wireless temperature sensor inside the attic. Hopefully the fan will keep the attic cooler and prevent the high 150 PSI I was seeing on the system.
Time will tell.
