You are correct it is the hammer at fault. The 125 requires very little return line pressure, Ie a 1′ return hose all the way back to tank.
The Volvo has small hammer lines, possibly 1/2 inch in places. Basically the oil cannot get away from the hammer quick enough so the working pressure has greatly increased.
The hammer then over heats and slows, sometimes to a complete stop. The hammer and machine are not compatible unless you do a 1′ return all the way into the tank. I have an out for these hammers should you wish to part X your hammer.