Oil pressure is made by bearing clearance within the engine and the oil pump turning with the rpms of the motor. An oil leak will not affect oil pressure unless you leak it all out or if it is the sending unit that is leaking. Normally, with proper oil level, the oil pressure, on an engine with a few miles, will be higher when first started and gradually fall a little as the engine warms up. The way I am reading your post, sounds like your oil pressure (with proper oil level) is around 25 with engine first started. When the engine warms up, it drops below that. The hotter the engine, the thinner the oil becomes. What weight oil are you running in the car? What brand oil filter? You can put a higher weight oil in the car to bump up the oil pressure a little but usually either too much bearing clearance or weak oil pump causes the oil pressure to be low. Also, check the wires going to the sending unit where they connect to ensure they are clean and are making good connection.
As far as the leak, check the front and rear main seals, oil filter oring, valve cover gaskets, etc to see if you can locate the primary leak. A leak that major should be pretty easy to find. Usually oil flows toward the rear of the engine from where ever it is leaking. Check under the car after it sits for awhile to see if oil is puddling on the ground.