Hi there,
Time to open it up and give a visual inspection.
look for any swollen capacitors or perhaps some corrosion near the power port. Snap a picture and post it up, we can look to. Could also be a cold solder joint, has it done it from the beginning?
HTH
GL 
PJ
also try a different AC plug for the power supply. with no other devices , if it is a power strip. 