|The PSU *may* be overheating, but the PSU fan also acts as an exhaust fan for the entire case. That means the hot case air is being sucked into the PSU & blown out the back.|
Considering the CPU temp was 68C & the shutdown temp is 75C, it's very possible the CPU is the problem. Removing the side panel generally doesn't help, in fact, it can sometimes make things worse because with the panel off, the case fans become useless. They need a closed environment to do their job. If you can, try positioning a small room fam so that it blows directly into the open case & see if that prevents the shutdowns.
You still didn't provide enough info about the PSU. The amperage specs for the +3v, +5v & +12v rails should be on the label on the side of the PSU.
Other than that, how many case cooling fans do you have & how are they arranged? Have you messed with the CPU cooling fan at all? Did you possibly bump it when you installed the new video card?