Hello Hannah,
Spotty data could be related to a multitude of things like (as you surmised) power or transmission-related issues. If you have the logs from the two “spotty” Shakes, I could give them a look and see what they can tell us.
If needed, you can find instructions on how to download them so on this page: Please read before posting!
Alternatively, you can download the entire content of the two Shake’s /opt/log folders via SSH, zip them, and post them here.