in the past i have come across a bad, contaminated bottle of 75/25 argon/helium while tig welding aluminum. i would swap the bottle out and everything would be fine.

the past few days i have been having intermittent problems with the cylinder i am currently using. i could weld for a few hours with no problems, then all of a sudden i begin to have problems, like erratic arc, my tungsten turning blue/black after the post flow has stopped, etc, sometimes lasting for 30 minutes..
i do not have any fans pointed in my area, and am far away from the open garage door and have curtains on each side of me, so i know its not any wind blowing away my shielding gas. i have been welding in this booth for 4 years.
then within time it will start to weld good again...

so my question is, if its a contaminated bottle of gas, is it possible to get good and bad spots within the cylinder, or once a tank is contaminated will it be all mixed with no good areas ? this is the first time i have had a bottle that acted good at times, then bad at other times..

just curious.... its driving me crazy....