Old thread but 3/8" fittings and hose will be more than adequate for a 1/2" gun @120psi. Not familiar with the earthquake gun but some guns have a debris screen below the air nipple. Be sure it is clean and not blocking some air flow. 3/4 "and 1" guns normally need 1/2" and 3/4" hoses respectively to get the most out of your tools. And like @huskihl stated be sure power adjuster valve is turned up on your gun. Mine is adjustable in both forward and reverse modes.
I built a tee with a small gauge that I tie in at the tool connector. That way I can monitor the air pressure at the tool. 90 psi at the tool is usually good enough. I originally built it, cause the air line at work were 130 at the tool and my die grinders didn’t like that.