  Instabilities described by linear theory characterize an important form of
wave-particle interaction in the solar wind. We diagnose unstable behavior of
solar wind plasma between 0.3 and 1 au via the Nyquist criterion, applying it
to fits of $\sim1.5$M proton and $\alpha$ particle Velocity Distribution
Functions (VDFs) observed by \emph{Helios I} and \emph{II}. The variation of
the fraction of unstable intervals with radial distance from the Sun is linear,
signaling a gradual decline in the activity of unstable modes. When calculated
as functions of the solar wind velocity and Coulomb number, we obtain more
extreme, exponential trends in the regions where collisions appear to have a
notable influence on the VDF. Instability growth rates demonstrate similar
behavior, and significantly decrease with Coulomb number. We find that, for a
non-negligible fraction of observations, the proton beam or secondary component
might not be detected due to instrument resolution limitations, and demonstrate
that the impact of this issue does not affect the main conclusions of this
work.
