Well, if you are finding that putting the co2 in the fridge helps then you should think about what temp[erature you are playing in. If it is hot weather and the co2 tank warms up in the sunshine (doesnt have to warm up much) then the co2 will be at a higher pressure than normal and the higher pressure will upset the balance in the valve between your valve spring, the hammer and the gas pressure. When it happens check your velocity - if the gas is higher pressure then your velocity will be dropping off (the valve can't open as much). You can try wrapping your tank in foam rubber or a similar material if thats happenning - we did it at the site I work at this summer and it solves the problem cheaply and easily.
Hope that helps,
Tom