Even with site hold on, it's normal to see the NAC and RSSI cycle on and off the screen every few seconds. If you just have system hold active, the drop in RSSI corresponds to when the scanner switches sites. As mentioned before, after processing data from the control channel, the scanner does some internal housekeeping before processing the next data packet from the control channel.
If Close Call is active, even Close Call Do Not Disturb, the RSSI will drop more frequently and for longer while the scanner checks Close Call.