sorry for the delay, last night I was frustrated, today I had errands.
so I pulled the fan/heatsink and gave them both a nice cleaning before putting things back together. blew out the whole case while I was at it. and it still shut down while rendering video. gonna look into “speedfan” and see what that tells me.
A) Always reapply thermal paste when you remove the heat sink. Thoroughly clean the heat sink and the CPU to rid it of old gunk. I use rubbing alcohol and a coffee filter (lint free cloth). Bit of a tedious task but be thorough and gentle with the CPU. For thermal paste I recommend Arctic Silver 5. Not all thermal compounds are created equal either and that is pretty good stuff (no I do not own stock in them or work for them).
Be aware that too much thermal compound is not good. You want just enough to do the trick. A tiny dollop on the CPU is generally enough. There are many guides on the internet on how to apply thermal grease. I’ll see if I can find a decent one…many get it wrong.
B) Not all heat sinks are created equal. Generally the ones you get with a store-bought computer are about the worst of the lot (they use the stock heat sink). For instance here is a chart showing the Intel stock heat sink. Notice at the very bottom is the stock heat sink and fan. Not sure about AMD but betting it is a similar deal.
Consider buying a new heat sink and fan. Pay attention to case size and what will fit…some heat sinks are huge. Also be sure to buy a heat sink appropriate to the motherboard you have. Not all heat sinks fit all motherboards.
C) When cooling consider total air flow. Where air comes in, where it goes out and what things it passes on the way. If air comes in the front, passes the hard drive then the video card and then the CPU the air is already warmed by previous components.
You should have intake as well as exhaust fans (more than one is normal). Usually there is an exhaust fan near the CPU (not the fan on the CPU). Check your case and if it has room to add fans then do that. Note that fans can make things noisy. With some effort and research you can find relatively quiet fans. Some cheap ones can be very loud.
If the case is too cramped or otherwise sucks consider a new case.
This guide shows the correct method for cleaning a CPU/heat sink of old paste and applying new paste (note they buy a lint free cloth but a coffee filter suffices just fine since it is lint free as well). Personally I think they use a bit too much thermal paste but close enough and this is the right idea.
TJunctionMax can get a lot hotter than the Core temperature. 100C+ on any temperature would be a good indicator of overheating problems.
It isn’t necessarily the heatsink. Some motherboards, for example, set the chip voltage too high. Unless there’s a fan failure (in which case the computer wouldn’t boot at all) even the crappiest heatsink is enough to run normally.