Update, after about six weeks after opening a new bottle of Ultra Black, and have had good success this go around.

Bottle was opened, but this time before closing it tight, I gave it a burst from can of compressed air (Falcon). Each time a new batch was mixed up the same procedure was repeated. I have now used the entire contents with no problems.

Thanks to Neal and everyone else, Jorge O you are correct, CO2 is not any better than air as a keeper.

Must admit, I am a little confused how a burst from a can of compressed air can make such a difference, but why worry about it, if it works.