Now lets see, I believe that if power control is trigerred based on level then the MS would take up large steps to reach the intended level in the middle of the power control window. the pow_inc_step_size and pow_dec_Step_size only come into play when the issue is with quality.
So for the fluctuations in the dedicated mode the issue may be wrong settings of power control window causing osscilations. What are the step sizes you are using could you decrease them and observe try having PC_Level_window width > 2*PC_Inc / dec _Step_Size.
Also try to set desired best quality to 0 so power decreasing based on quality is not trigerred.
As for Idle mode fluctuations so severe the only issue that is can think of is a multipath area where you are getting reflections from various paths.try in some open area LOS if the issue still persists then probably the best bet is a hardware issue.
As in idle mode you are measuring BCCH DL power and it is transmitting at max power.
Also try changing your test phone it can be faulty and giving you this abnormal reading.