Bill Beam wrote:

I've got a CO2 Tank/Reg system. I keep the out going pressure at
5psig into a needle valve. I have the valve set for 2 bubbles per
minute and it stays like that all day. Problem is - when I get up in
the morning the bubble rate is very slow - less than one bubble per
second. Pressure is the same. Resetting the rate with needle valve
works fine. Why would this be happening? This is one of those $25
needle valves from Foster/Smith. Should'nt it stay adjusted? Thanks
for the help.

350X Rider already suggested temperature as a cause. Check that first.

If that's not it, check the tank output valve-the knob on top of the
tank. The directions on my CO2 regulator said to just crack that valve a
bit, which unfortunately can cause unstable gas flow. Open that valve a
full turn at least.

Also consider setting what you called the "out going" pressure closer to
10psi. That may stabilize things for you.