Strange behaviour in Observation 306365 at my station. At beginning of the pass, the signal is received at correct frequency, then it jumps out by about 20kHz.
I see occasional slant on UNISAT-6 that may suggest TX drift; however, your problem seems like frequency error in the SDR dongle. See for example obs 306338
I also think it is a problem on my side. From the log (/var/log/syslog) there is nothing strange around that time. Is there another place where I can find something?
BTW, dongle is a NooElec (that with aluminum case).
If you have access to other sdr, try it in order to make sure if it is an sdr problem or sth else.
During an observation, try to check cpu of the rpi (? do you use raspberry pi, which version?). To check cpu you can ssh in your rpi and run htop. (If it is not installed then run sudo apt-get install htop).