intermittent mechanical? power-up failure

[nobody99]

I upgraded my cpu, ram and mobo about 1 year ago and reused the case and psu and old drives.
recently it would not power-up - no lights fans or beeps
I checked that all connectors are seated (they were) but discovered that the problem seems to be mechanical
It will power up for several days, then fail. I bump or move the computer and it powers up.
Once powered up it runs normally for hours until I shut it down.
I'm guess psu or motherboard. I'd like to rule out one and replace the other - hopefully it's the PSU
I welcome all informed guidance.

 

[pcrepair]

If you have a spare, known-good PSU, try connecting it to your system to see if the problem persists.
Use a PSU tester if available to check the voltages and functionality of your current PSU.