When the pump and filter is off does the pressure gage go to zero? these gages are junk really, try a new one before going crazy.
Also how old is the sand, it may need to be changed if it came from a discount store.
Put the valve on recirculate it the pressure any different? test at the pool returns also
new system, the gauge is good if i put it on low speed pr15
could it be to much sand or to little