We have a 621F payloader, that when it warms up it will make a clicking noise, and we will immediately lose steering and lift. By moving the bucket, we regain hydraulics, but we will lose them again by just steering the machine or lifting. Is it the check valves? A spool valve? We do see a little scarring on a spool valve. But would it cause all these problems? Our pressure is running around 1000 psi on the main valve bank under the lift between the front tires. It clicks out around 900 tops. When machine is cool, it works great!
It sounds like you have a hyd. lockout problem. Check your relays and fuses, sense you can recreate it by steering check steering pressure also.
Kind of sounds like the splines on the pump drive are stripped.