Maybe 'that' head needs a higher octane if nothing else is wrong. A ping or knock when accelerating?
That's a good price for a complete head. I'm skeptical that all the valves/lifters/rockers/springs/clips/cam are new too though.
Have you mailed back your 'old' one yet? If not consider keeping it and finding a complete salvaged/damaged one to mail back - if they take damaged heads.
Member of the Dropped Valve Seat Club.
Replaced #4 Piston, Rings, Rod+Bearings, AERA Re-Manufactured Cylinder Head and a light Hone. Then CLEANED the hell out of everything.