Compressor bogs down on start up

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?