Unfortunately, this seems to be one of the more common problems with Milwaukee regulators. I assume that your CO2 cylinder is still full of gas (or has been (re)filled recently).
The needle valve tends to be very floaty, and when you think it is set, you will come back a few hours later to find that the bubble rate has drastically changed (as you have already found out).
The problem that you are experiencing has nothing to do with the solenoid (I assume that if you unplug the solenoid, gas no longer flows), but rather the needle valve.
You can try replacing the needle valve, or if you are looking for a completely new setup, you can try DIY-ing your own pressurized CO2 setup.