Muzzle blast can cause erratic readings, as well as pieces of unburnt powder being detected by the instrument. Sunlight can be problematic, too. Occasionally the odd angle of sun or a little reflective glint can cause a momentary inability to be accurate. Magic? Seems like it.
Sorry I can't be more help, but in general:
Set the instrument as far as possible from the muzzle of your gun.
In direct sunlight set the diffuser screens properly over the sensors.
In open shade with clear blue skies, try with and without sensors and see which seems to work best.
Under partly cloudy skies where the sunlight is intermittent, wait a while and hope the weather changes.
Under even, light overcast, count your blessings. That is the best light for your chronograph.