  We obtained ultraviolet and optical spectra for 9 M~dwarfs across a range of
rotation periods to determine whether they showed stochastic intrinsic
variability distinguishable from flares. The ultraviolet spectra were observed
during the Far Ultraviolet M~Dwarf Evolution Survey
\emph{Hubble~Space~Telescope} program using the Space Telescope Imaging
Spectrograph. The optical observations were taken from the Apache Point
Observatory 3.5-meter telescope using the Dual Imaging Spectrograph and from
the Gemini South Observatory using the Gemini Multi-Object Spectrograph. We
used the optical spectra to measure multiple chromospheric lines: the Balmer
series from H$\alpha$ to H$10$ and the \ion{Ca}{2}~H and K lines. We find that
after excising flares, these lines vary on the order of $1-20\%$ at
minute-cadence over the course of an hour. The absolute amplitude of
variability was greater for the faster rotating M~dwarfs in our sample. Among
the 5 stars for which measured the weaker Balmer lines, we note a tentative
trend that the fractional amplitude of the variability increases for higher
order Balmer lines. We measured the integrated flux of multiple ultraviolet
emission features formed in the transition region: the \ion{N}{5}, \ion{Si}{4}
and \ion{C}{4} resonance line doublets, and the \ion{C}{2} and \ion{He}{2}
multiplets. The signal-to-noise (S/N) ratio of the UV data was too low for us
to detect non-flare variability at the same scale and time cadence as the
optical. We consider multiple mechanisms for the observed stochastic
variability and propose both observational and theoretical avenues of
investigation to determine the physical causes of intrinsic variability in the
chromospheres of M~dwarfs.
