ICP-OES 5800 Chromium falling out after couple hours

Since you have calibration standard concentrations in the ppb range, are you making your standards fresh daily?

I prepare them weekly, more often if I run out, but this issue persists even with a standard set made the same day, and there have been instances where a prep performs better on the second or third day than the first.

If the Cr is part of multi-element calibration standard, you might want to try preparing a single-element calibration standard with just Cr to see if that helps the stability.

I don't exactly have the time to do that. If the standard mix were the issue, however, I would assume the problem would be more consistent, or carry over between days when I don't prepare fresh standards. This is not the case. Cr will read ~5000 c/s at calibration one day, creep up to ~6-7000 c/s within a few hours, then go back to ~5000 c/s at calibration the following day, all without any new standard prep. That, or it comes in at 10,000 c/s at calibration and stays there

I'm more inclined to believe that it could be a plasma or argon issue, based on the patterns I'm seeing.

If it were a plasma issue, I would think it would affect more than just Cr.  That being said, I would suggest you add an internal standard to your run, if you don't have one already. Then you can monitor the Yttrium or Sc internal standard and look at stability. By ratioing Cr to the internal standard, it may help to normalize the results for Cr.  You could also add the Ar 420nm line and look at it. 

Already have a Yttrium internal standard. Even in longer runs, it stays between 97-101% recovery. It will drop lower with particularly dirty samples, but it doesn't carry over, and my CCV is almost always at 99%.

As for Argon... I'd have to redo all my IEC standards to add it, but for today I could at least keep an eye on my gas flow and pressure over time, as well as instrument temp. If those drift over time, it might provide some insight You are right, though. If those were the case, it should affect more than just Cr.

Additionally, there's an ICP-MS hooked up to the same argon line as my machine is, and we both use the same standard mixes. They're not having the same issue.