Inspiron 620s malfunctioning

I have an old inspiron 620s with core i3 sandy bridge. The system was not used for a long time, but a year ago when I switched it on, it worked fine but suddenly shut down itself and the power button showed an amber light blinking. A week ago I thought to look into it, when I switched it on, it turned on, worked fine and shut down itself again, then the weirdest thing started happening, the power button would turn white, all fans (CPU, GPU, PSU and the fan in front of HDD) would turn for a second or two and then stop suddenly with amber light on the power button. Once the amber light showed up, it'd turn white again and this loop would continue for eternity.

I changed the CMOS battery, same thing happened, the system turned on for a few minutes (enough to log into my desktop account) and then shut down with the eternal loop of "Dormammu I've come to bargain". 

I tried the little trick that involved disconnecting the three wires from the USB ports of the front panel from the mother board as discussed in a forum here discussing 'power button problem'. No good, it still came to bargain.

I checked the green time stone (green light on the back of PSU) which my pc already possessed. At this time, I should also inform that despite the green light, my PSU was repaired by some third party workshop two years ago after it was turned on at 240 V Wall socket while being set at 130 V (that's when the sorcerer crashed it's lamborghini, a foolish mistake) but it worked fine once the PSU was repaired.

On different occasions the eternal loop would stop and the amber light would blink forever. What I have noticed is that when I start the pc after a while, it'd start for a few minutes and then start the 'eternal loop' of fan turning sorcery. If I turn it off and connect again within a timespan of say, 10-15 minutes, it'd continue the loop, but if I start it after a few hours, it'd turn on normally and then start its eternal loop.

If changing PSU is the solution, kindly help me with which one I should buy as I can't find the 250W slim ones for this model here in India, but the 250W ones for Optiplex are available but I don't know if it'd be a good bargain for my sorcerer.