This might be a good time to step back and look at a few simple things as well. Not knowing how you have checked but I might look at the power to the solenoid also. Some timers can be funky and not work as they should. When the bubbles stop, have you tried plugging the solenoid in directly so that power to it is certain?
Yep. Unscrewed the solenoid body and adapter and plugged it into a standard outlet to see if I could hear the faint "click" of the spring...nothing.
I sheepishly admit that I think I know what the problem is/was. Somehow I forgot to put a check valve in between the bubble counter and the needle valve. I have one in between the diffuser and the bubble counter and must have somehow overlooked this huge error. I can only assume that a small amount of water seeped back into the solenoid...though I have not seen any physical evidence of this. This would explain why the solenoid was "faltering"...what a dumb mistake. Off to order another solenoid...though on a brighter note I found them for $10 bucks!