That's just indicative of a PID system going into oscillation. The PID variables were set at a higher temperature, so when it's run at a lower temp, the system oscillates like you see in the graph. Pretty safe to say that 175 is approaching the limit of the controller's ability to keep things running. Probably could be fixed by increasing the D (if it was even available to set), but then the system may not keep temps well at higher settings. PID stands for proportional–integral–derivative, very common in the process control industry. LOTS of stuff written on how to tune them.