Batteries can have intermittent internal open circuits, if this exists then the battery is dead, however sometimes the 'open' is a heat/vibration/current sensitive high resistance condition which can indeed cause flickering.
If it's a Varta, replace it as the likelihood of a 10 year old battery exhibiting this condition is more probable.
Listen to Padgett here on this forum and get yourself an AGM ( Absorbed Glass Material ) battery.