My Atlas Copco GA-7FF starts, runs about 30 sec before bogging down.
By bog down, I mean compressor side slows way down to a stop while motor is still able to turn enough to make belts squeal.

This compressor is a 2004 with 18000 hrs but is new to me.
On receiving the compressor I changed out filters and oil.
Oil shows in the sight glass about 3/4 up the glass.
Screw turns freely when belts removed.
Belts are tight, with maybe 1/2 inch of deflection when pressing with some force.

What are all the potential causes and where do I start troubleshooting to isolate the problem?