How to determine the source of sub-nm periodic error in NIR spectra

I've run into an issue collecting measurements on deionized water in the $1050-\pu{1200 nm}$ range. To determine the sensitivity of my spectrophotometer in this range, I've acquired measurements on pure deionized water in double-beam mode using two identical quartz cuvettes. I have taken measurements at several time steps near $\pu{1064 nm}$, from $0.1-\pu{100 s}$ per time-point and constant $\pu{0.1 nm}$ per step and $\pu{2 nm}$ bandwidth. I keep finding that the baseline standard deviation (SD) does not improve beyond $1.4 \times 10^{-4}$ Abs.


This value is technically below the acceptable qualification values of $2 \times 10^{-4}$ Abs in that range, but I'm trying to determine if this is the ultimate noise level of the instrument in this range or if these periodic variations are due to a mechanical issue in the monochromator like a lead screw, or due to environmental or electrical line noise of some kind.


[![enter image description here](https://i.stack.imgur.com/fO1Hc.png)](https://i.stack.imgur.com/fO1Hc.png)


Any thoughts are appreciated.


