What year model is this 1680?
Here is a thread with pictures
on another site of my 1680 that I changed out the relief valve on and installed a gauge in the pressure line to diagnose issues just like yours when they come up. Now that it has been several eyars running with the gauge and having to use a few times to find a problem, I would move the gauge to the sheet metal just outside the cab, over the valve stack so I can read it when operating the machine myself. As it is, the gauge location requires a helper to activate the machine so I can read it.
But you must know the pressure on the pressure line going into the valve stack before you can make any kind of repair worth while.
My 1987 year model had a relief cartridge in a block just a few inches away from the pump. The cartridge went junk and a new one was stupid cost. So I bought a then 39 dollar Prince adjustable relief that is on the shelf at pretty much all hydraulic stores.
It could be the relief on your machine has given up. But you will only know if you use a gauge to check pressure. Sometimes guessing can get lucky and fix the problem right away. But more times than not, it just cost a lot of money replacing good parts for nothing and leaves you telling yourself "I should'a just used a gauge to start with".