Is My Power Supply Unit Dying?

[john]

It sounds like a simple solution would be to add an extra fan to blow air through the psu.
DONT open the PSU, just fit it to the outside of the psu case..

Graphics cards do run hot, not a problem.

Steam relies on an internet connection; if that is slow or disturbed

The last couple of months, it seems to be getting slightly slower at turning on
maybe you have installed something that runs at start-up?

Run Glary utilities to repair registry tc.
http://openhardwaremonitor.org/ to check temperatures
run a speed test on your internet

check the other fans they may need replacing if they are old.