check packing and flow resistrictions
Its sounds like a conventional packed plunger pump (piston). This style is very good at dead lifting water out of a tank, can create lots of pressure but tend to be a lower flow design compared to other styles like diaphram pumps or vane types. These pumps have packing so just like your prop shaft, a little leakage through the packing keeps it lubricated. If its too tight, you can guess what happens. Depending upon what your using this water for, you could try some water soluable lubricants like glycerin which may help till the packing seats or wears a bit. If this is for drinking water, don't use anything other than a food grade glycerin. The other consideration is this is a true positive displacement pump so if it has any flow restriction on either the intake or discharge side, it will create a lot of resistance. Before messing with the packing ensure your lines are the right size. If you used conventional pipe sizing, it will be too small. Pressure losses are calculated based upon Pi X the rated flow of the pump. So if it's rated at 10GPM the lines have to be sized for ~31 GPM. Hope this helps. Mac.
