How is the pump primed??, some pumps have a foot (non return) valve in the bottom of the pipe to keep it full of water when the pump stops, if its only loosing the prime when pumping then it may be pumping at too high a flow resulting in insufficient NPSH, see below, can you post a link to your pump.
All pumps require a NPSH (net positive suction head) of ~ 0.5M (or so), it will be in the pump manual.
The NPSH is the atmospheric pressure (absolute) - the suction lift - the suction pipe friction losses.
Normal atmospheric pressure (absolute) is 1013mb, at 800ft elevation it will be ~ 992mb, 9.92M, suction lift is 25ft, 7.6M, if your 1.25ins pipe is 1ins ID, then the friction loss through 25ft would be ~ 1M when pumping 50LPM, so the NPSH will be 9.92-7.6-1.0, 1.32M which should be adequate, (pump details will tell).
Theoretically the negative pressure will be the height from the pump suction to the well water level, say 25ft=7.6M=0.76bar,11.0PSI with no pumping, and 7.6+1.0, 8.6M,0.86bar,12.4PSI when/if pumping @ 50LPM.
