I have seen similar fluctuations in some of the streams where I have Hydros21 sensors. Typically, step changes > 100 µS in 5 or 15 minutes only occur during runoff/snowmelt events. In the winter, after a snow event with road salt application and subsequent melt I have seen > 2,000 µS fluctuations in 15 minutes as the salt pulses work their way down the streams. In the non-snow season a rain event can quickly dilute the streams that have base flows with high conductivity, dropping very quickly in 15 minutes.
If there is no active runoff/snowmelt I tend to see more stable conditions without such rapid fluctuations.