Low oil level.
Fill reservoir. Look for leaks. Drain and replace oil.
Water in oil.
Check filler breather on reservoir. Heat exchanger may be cracked.
Wrong kind of oil being used.
Use oil recommended by manufacturer.
Air leak in line from reservoir to pump.
Tighten or replace suction line.
Kink or dent in oil lines.
Replace oil lines.
Worn pump shaft seal.
Clean sealing area and replace seal. Check oil for contamination or pump for misalignment.