seriously, it could a be contact sa alternator. Try to unplug the alternator contact (ung plug na merong 2 or 4 wires), clean the prongs (if you have a spray on contact cleaner mas madali) or maybe WD40 would help.
It could some loose contacts. The battery would only supply power enough to crank the starter pero when running, all other equipments are supplied by the alternator. So mukhang nahihigop ng light mo yung power minsan - signs of loose contacts or dirty contacts yung ganun because of high resistance.
loose contact yan. nangyari sa akin yan nun kapapalit lang ng battery ko. pag nag power window ako baba ng 8volts yun voltage, kahit signal light bumababa pa rin.
sorry sobrang late ng reply.. ngayon lang nagkachance mag-internet.
tama kayo loose connection nga sa battery. umuwi kasi ako ng late nung sat dahil sa sunod-sunod na exams. tas pagdating ko sa kotse ayaw magstart. in-on ko yung cabin light tas ginalaw ko yung batt terminals. yun nga maluwag. hinigpitan ko na lang. salamat po sa inputs nyo!