The diaphragms are probably getting old and stiff.
You could Jerry rig it by adding a second O-ring under the solenoid as a temporary fix.
If you replace the valves I recommend you buy the Irritrol 311A. The ports are a similar width apart.
You could use the same brand valves. They're not the greatest but you could do worse.
http://irritrol.com/valves/valves_311as.html < cut n paste the link.
P.S. The valves have to be at least 5 inches apart for the 311's to fit side by side. Otherwise you'd have to stagger the height.