|"...was getting a lot of browser crashing and numerous other problems and decided to upgrade the OS. Now it is worse..."|
It shouldn't matter what OS is used.
"Probably either the cpu or video card getting too hot."
"will recheck and look at bios temperature later."
That's the first thing you should check - the cpu temp - when you get symptoms like you've been getting.
You can determine what cpu you have on both the Intel and AMD web sites by running a program, if you don't know the exact model part number, and you can search the web to find mentions of what typical idle and load temps people are getting for that cpu.
"This machine hasn't got all the bits from other machines in it. I only replaced the motherboard in this one."
When you're dealing with computers you didn't put together yourself, then it's quite possible this applies, especially if the cpu was upgraded from what the system originally had....
The cpu fan is inadequate, or the cpu heat sink is inadequate, or both.
If you have another cpu fan or cpu fan heat sink combo that can be mounted on the heat sink or cpu that has more cfm capacity (e.g. the same physical size fan will have a higher current rating and/or spin faster if it has a higher cfm capacity) or a larger heat sink fin surface area, try that.
The cpu fan is installed upside down.
"I put the thermal paste on according to a youtube video but i will recheck when i have time. "
It's good that you did that, but that's usually not necessary, and it's difficult to see whether the heat sink is actually sitting flat on the cpu when the mboard is mounted in the case.
Personally, if the heat sink is not loose on the cpu when the heat sink has been un-clamped, before you try twisting the heat sink back and forth while pressing straight down, I've learned I don't need to mess with re-doing the thermal pad or paste or grease, my reasoning being that if it's not loose, then the thermal contact between the two is fine. I've never had problems when I assumed that.
Removing a heat sink that has become bonded to the cpu as if it were crazy glued because of the hardening of inferior thermal compound is a nightmare (thermal pads don't do that in my experience; thermal grease - pure translucent silicon compound with no additives - never hardens). In that case, you have no choice except to pull straight up on the heat sink / cpu combo because you can't release the ZIF socket lever when the heat sink won't come off the normal way, you risk damaging the cpu pins or the ZIF socket when you do that, and I have NOT been able to get the cpu un-bonded even when the assembly was removed - I could see no way of doing that without damaging the cpu while doing so . If it's firmly bonded when you unclamp the heat sink and twist the heat sink back and forth while pressing straight down hard, don't bother to try the remove the heat sink, unless you want to change the cpu.
"I might also reorganise things so there is more airflow around the backs of the machines .."
AMD proved in tests that if the system needs a cooling fan, the best place to mount it is on the back of the tower case as high as you can mount it, blowing air OUT of the case - the support rib(s) and central non moving center part towards the outside of the case.
Cheaper cases tend to have not enough total hole surface area for a fan to have decent air flow where a case fan mounts. I've cut out the metal on a few cases because of that, but that's not easy to do, and on one case I had to install a fan grill over the open hole because the fan was too noisy in the open hole. In retrospect , a better way is to use drill bits larger than the holes to drill out the existing tiny holes, or drill more holes. Either way, you produce metal bits, so it's better to do that when the mboard is not mounted in the case.
"I do have th computer on too long sometimes."
That shouldn't matter.