APC back-ups CS 500VA gone mad

[reb0rn]

I have this UPS almost 6 years (last year bought new battery) so far so good, but the problem started..

Today it gone mad while there was fine power current input it just gone to battery mode for 10sec and after that it stared verryyyy fast switching (PC instanly go on/off/on...) from battery to input current (10 times per secand),, than i tried to unplag it from power for 20-30 secands (and hear that "beep" like current being let out from capacitor) and it started to work again

I think something inside is failling being quite old, i have friend electrician he could take a look..., any advice what might went wrong?

I also notice now while its on battery it can hold only 40sec and than it just turn itself off, i tried unplaging PC, use 80w light bulb all the same it hold only for 40sec no matter the load on it even if nothing connected it hold the same ~40sec... also if i abuse it it start to fast click again (every time first it switch to battery mode for 10sec and only after that clicking start)
battery is fine fully charged 13.6V ...