There's a number of things you might try.
You may want to get an inline regulator, this should make the pressure that is fed into your marker a little more consistent.
However, probably the best thing you can do is switch to compressed air. It's a far more stable gas than Co2, and it is effected a lot less by atmospheric conditions. All air systems come with a regulator that controls the output pressure. I think that may be your best option. But it ain't the cheapest option, sorry.