Rainbird, Toro, Irritrol, Hunter - all have similar problems.
Other posters have addressed the obvious problem - the diaphram.
Another possibility is an obstrution (sand, dirt, ect.) in the small passage between the lower chamber of the valve and the upper chamber. An obstruction would operate to the same extent as your solenoid does - it closes the passage allowing the valve to open and stay open.
Yes, do call your local professional.