I don't have much experiance of different markers nor do I pretend too. however at the tourney I played at the weekend my shocker (on CO2) varied its velocity by plus or minus 5 fps ( as it was set to 263 it was ok) a lower end marker waried by up 20-30 fps on air, now I admit that co2 is invariably more unstable due to teh nature of the beast as temperature will play a large role as it changes states, however ( I know you saw that coming) I think from a physics point of view a lot of test are flawed my shocker ran fine on air ( about the same range of velocity change) I think more that the gas it is the reg, barrel, bolt even paint that will cause fluctuations in velocity,as for the cloud, an anti syphon is the way to go to fix that, if that doesn't work I'd suggest a check of your o-rings.
John
John