The only two things I can think of are some kind of electrical problem (short, power surge) that keeps frying the electical end of the solenoid, or something mechanical in the valve (rock, corrosive material) that effect the mechanical end.

there is the possibility you were treating the symptoms and not the problem when you changed the solenoid. debris in the area below the solenoid will have the same symptoms as a failed solenoid. One way to test this theory would be to take the solenoid and the cap off, flush the areas with clean water, then re-assemble. the solenoid has a plunger and a small spring located in the end that screws into the cap (make sure you don't drop the spring). pull the plunger out and make sure there isn't debris inside the solenoid. also make sure there isn't debris in the area directly below the plunger (small openings with edges; don't damage by sticking a knife or other tools into this area).

Thanks for the reply, I think you're right. I think I may have a problem with the wiring; I noticed that this is the only solenoid that makes a loud buzzing sound when it does actually work, all the others that I have don't. Still not sure what makes it work sometimes and not other times, but the solenoid itself is definitely not the problem. I did flush and clean the valve completely.

Thanks