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.
Yes, that could have been the problem.
Or, the solenoid just failed. It happens.