1996 Ford Probe very low idle and sputtering

i drive a 96 ford probe gt 24valve 6 cylinder non turbo and my check engine light came one. i replaced all wires plugs, left and right gasket covers and head gasket. it fixed the check engine light problem for a while but recently when im idle my car will shake like its struggling and idles at 500 rpm and starts flashing the check engine light. however when driving the problem does not exist. i have tried idling and giving enough gas to wear it idles at 1000 rpm and the car sits fine. its only when im completely idle does the car shake and the check engine light starts flashing.

help is much needed =[