It is normal for the ignition timing to vary while idling. This is from the ECU controlling idle speed by varying ignition timing.
Large variations in ignition timing can be caused by a faulty TPS sensor. A rapidly changing TPS signal causes ignition tip in retard.
In this cause graphing TPS and ignition timing will show rapid changes in TPS at the same point the ignition timing changes for no discernible reason. The TPS must be replaced.