Well 12fps variation is not bad right up to around 101 bar - appears reg is nominally set at 95 or your gauge is not incredibly accurate.
As for air consumption - you may need to tweak hammer spring and perhaps hammer opening, the factory setup tries to give its best over a wide range of pressures, working well at full pressure, and opening further/longer when the pressures drop, so now the gun is regged and the pressure is permanently low too much air may be being wasted.
**WANTED**: WEBLEY PATRIOT MUZZLE END; Any Diana/Original mod.50 parts, especially OPEN SIGHTS