non-inverting current sense op amp output unstable during the measurement
Do you have a scope?
Motors tend to draw current in spikes.  This means that the voltage on R19 would be going up and down very rapidly.  If so, the LM324 may be hitting its slew rate limit.  An RC low pass filter could make the LM324 happier if that is the case.
Depending on the meter you are using to make your measurements, the large AC component on the &quot;unstable point&quot; may be making it inaccurate.  Increasing C16 will help this.