To try a different power PSU, will help to eliminate the problem.
If it will work with a new PSU, problem solved, if not, lets keep looking.
PSU deteriorate over the time, like anything else.
Your PSU may have high/low spikes, that happens when capacitors starting to fail.
The longer the PSU works, the WATT's are getting lower.
Cheaper PSU's are using cheaper parts, so it dies quicker,
that's why u can see 500W psu for $30 and for $150 as well.
Even if the PSU turns on, it can be the problem. And often is.