Its sounds like the solenoid is too weak and isn`t able to close against the pressure from the pump/mains water supply side.Effectively its being forced open as the pressure builds up. If it works for a while after you release all the pressure from the lines it is a solenoid pressure issue.
I`d recommend using anti drain valves on the mister heads to prevent the lines draining, you`ll get an instant on/instant off mist instead of it ramping up to pressure and ramping down as the lines drain.. no dripping nozzles either.

Thank you for your help. It's another brand-new solenoid but I guess it could be the problem. Very frustrating to have to deal with this for so long after the original purchase! It will be at least another week before a new solenoid can come in if I contact the company and they send a new one. The ones at my local store don't seem to be strong enough to work with this valve/timer. We tried one, anyway, and it worked only with the water off and not even once with the water on. The original one first sent was completely bad and didn't respond once even with the water turned off. This one works perfectly for up to several hours and then just quits.

I haven't seen the anti drain valves. Is this the type of thing you are recommending: http://www.mrdrip.com/eshop/catalog/153.asp

That's really expensive for each of the misting heads if that is what you are talking about, but it would be really nice to have that feature on the misting system.

I guess it depends on the mist heads you have, i use netafim coolnet foggers and they have small ADV`s inline to each head, its an egg shaped unit with a spring inside that pushes on the internal tube.
They come in different pressure ratings, the ones i have open at 58psi and close at 26psi.
When the solenoid opens it pressurizes the line from ~26psi to 58psi at which point the heads instantly fire up (no ramping of the spray). When the solenoid closes, the line pressure drops to 26psi and the ADV automatically shuts off the flow..no drips.
With currency conversion the adv`s wouldnt cost more than $1 each. A 4 way cross head assembly,adv and hanger would probably be around $6. Good for a 5ft circle coverage
Cold water inlet solenoid valves from washing machines are very reliable (and sometimes cost nothing), they run on mains voltage though so you`d need to mount them indoors out of the weather for safety.
To run one with your timer you`d need probably need to use a 24v coil relay with mains rated contacts. Mine runs quite happily with cycle times of 1 second on/60 seconds off although i generally use 3 seconds on/3 minutes off to give the solenoid an easy life.

If you have broadband i`d recommend taking a look at the greenhouse misting video on the netafim website, its a 15MB wmv but shows you exactly what they do.
www.netafim-usa-greenhouse.com/Micronet_Fogger_video.wmv

Thank you for your help. I just have a cheap misting system from Sam's that is intended for patio areas that I strung up around my greenhouse and it works really well to cool things off which is the goal for a misting system in there. It has about 15 misting heads that run on a tube that I have connected to a garden hose. I have the garden hose connected to the valve/timer (it's battery operated) and then the filter goes before the valve and to the faucet.

I need to edit to say I have 30 misting heads, not 15.

I have a few around the patio, i took a pic when i tested a single head mounted vertically. It delivered a lot of mist with 75psi water pressure.
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