Inspiron 531 - unexpectedly fails to turn on

Can someone advise pls?

I've enjoyed relatively trouble-free use of an Inspiron 531 for a couple of years, but very recently the machine started hibernating after being left unused for just a few minutes. At resumption, trying to bring it out of hibernation became a struggle. Power clearly on, blue light at the front, fans working but no video. I have found various ways of kicking the video back into action, which would happen after perhaps 10mins of fidgeting around, at which point normal use would resume. I started to feel the system would benefit by maintaining good and very regular behaviours of closing the machine down and then booting back up. Then a couple of weeks ago after closing the machine down the previous night, I tried to re-start and NOTHING. Touching the Power button at front had absolutely no response at all.

Green light on back panel tells me the machine senses power coming in, but there is no light at the front at all ( no blue, or amber ). Taking the sides off I can see a steady amber light on the motherboard. I've read some other posts retelling similar issues on Inspiron 531's most of which suggest a possible fault withe power supply.

I have therefore tried disconnecting power cord, unplugging the 24pin power connector from the MB and on this 'jumping' a green input to a black input using a paper clip. On reconnecting the power cable, there is a response from the power cable fan, and a hum from elsewhere which I think tells me that the power supply works and is OK - am I right so far ? However with power still on when i connect the 24pin connector back to the MB everything goes quiet again.

Is this telling me the motherboard is knackered? or is it MB and power supply ?

If not the power supply then what is the issue ?

gardenermike