Pain in right foot – instep and arch

For the past 3 months I have been experiencing pain similar to an earache on the instep of my left foot, occurring in the early hours of the morning, preventing sleep. Also difficult to put my foot (especially heel)on the ground first thing in the morning. Also hurts to move foot up and down or stretch foot out. Also have cramping in the foot during the night and sharp stabbing sensations sinilar to an electric shock occassionally during the day and night. Diagnosis is plantar fascitis. Have tried supportive footwear and orthoheel inserts in shoes. Foot has also been taped – nothing seems to relieve the pain. Physio says pain in instep region is more commonly caused by chip or fracture to one of the small bones in that region of the foot, however, nothing has shown up on X-Ray. Any ideas on what the cause may be and how to better treat it?