If you have your water bypassed to the tank,you will see if your unloader is bypassing water all the time. Low pressure means low volume. If you are losing water back to the tank, you are losing pressure there.
Paul has hit the nail on the head. Isolate your pump to see if your pump is the problem. Hose chatter is almost always the check valves. If you've had the pump for several years, and you have determined that the pump is the reason for the pressure loss, then I would replace the seals and check valves.
Often, you can purchase 3 kits that will have all the brass, seals, and orings. I would reccomend this for an aged pump.
You will probably find that your unloader is also sub par if you are having all these other problems.
Water in your pump crancase oil is a sure sign that your seals are bad. You will also need to inspect your plungers. A small thin pencil type of line is a crack. If you find this, replace also. Depending what pump, there is sometimes an oring on the plunger rod, or a plunger nut that has an oring and teflon seal that holds the plunger on. These can be the cause of water into the oil.
Where as you are circulating your bypass water, you are circulating at an increased temperature that will get hotter with each minute. After 5 minutes you can do damage to your pump and unloader.
Rout your bypass to the tank to prevent future damage.
Good luck with your repair.