Your problem could be air, also could be a worn pump or some other anomoly.
Best place to start is to tee a pressure guage, preferably a 5000psi guage into the main hyd line at the pressure side of the control valve. See what kind of pressure your getting. You should get around 3200 to 3800 psi. Anything less and you have a pump issue most likly.
If this low pressure only happens when you first start up and improves after running for a few mins then forget this and go on.

Do the simple things first such as:
Fluid level
Check Filter and sump filter (could be an inline filter)
I've also had the sump screen inside the tank come loose and get in front of the pickup tube and cause these type of problems.
check the suction line from the tank to the pump and make sure it's not collapsing period. If it even appears to be collapsed even slightly replace the hose.
Check all hose clamps, and check lines for cracking , rubbing, scrapes, cuts, etc.

Ok one other place to check is the pressure relief on the control valve, remove it and check the spring on it, if broken replace the spring or valve. Check for gumminess, if sludge or varnishing appears to be present clean it.

I know this seems like alot to do but there can be a variety of things that can cause hydraulic issues such as this.