I would expect it to be capable of what you describe, but there's a lot going on in your description and the current state isn't so clear, nor is the information about how a failure appears when it is happening. Is the device happily polling and then you open the chart you are describing, then things go off the rails?
As far as RAM settings, I would think for the amount of RAM you have specified the sweet spot will be between 650-850 and no swap space. Keep in mind the other things on the machine can impact what's available to Mango, so a light operating system is advisable (are you running headless or with a desktop?).
some details I left out:
You left out which version of Mango when Jared asked.
There are other SBCs in the intermediate space to the i5 8GB machine you make references to.
I would avoid using "ON_CHANGE_INTERVAL" logging if you're hitting the memory ceiling (I wouldn't do an interval average, either), but I wouldn't expect that to have a big impact unless the interval is very long (as there will be a lot of cancelled logging tasks hanging out in the timer until their time comes). You could post screenshots of the tabs on the
/internal/status.shtm page. Perhaps one of them will reveal another place that memory could be going.
I am running 300 virtual points on a compute module right now, polling on 200ms (doing ON_CHANGE_INTERVAL logging) and it's downloading data and charting fine. Did you design a dashboard for the frequently updating chart you describe? Can you share that code?
One of the distilleries is running experiments and being able to view 15-20 temperature sensors at a given time and have them graphing every 1-5 seconds is imperative.
Are you using the NoSQL module?