My astra mk4 estate has a niggly prob that more annoying that anything.

Drives but fine then battery light comes on for no reason now only adverse effect of this seems to be the steering goes rock hard. could be on for 20 secoinds to 5 mins then goes out. then steering eases again. managed to check voltage while light come on and seems good at 13.5v or there abouts so dismissed my thought of reg on its way ???? changed battery few weeks back with 2 burnt out cells. and same on new battery. can go days without it happening then few times in 1 day

Any ideas ???