There may be a tiny hole in the diaphram or it may be worn out. You also might have a rock in the valve holding it open. Unscrew the valve and flush it. If it still occurs replace it with an Irritrol 2400 valve (same as the old Hardies) found at sprinkler warehouse. You only have to unscrew and take out the old guts and replace with the new iRritrol.
Thanks
Ray
Rays Above and Beyond Automatic Lawn Sprinklers