Is it normal for a 27" iMac to have CPU core temperatures around 95C under 100% CPU load?
Reaching a temp of 95°C is not surprising if the CPU is under heavy load for a prolonged period.

Prime95 can be useful for determining stability, but I'm not particularly a fan of it as a troubleshooting/diagnostic tool. The reason it's popular  for stability testing is because users will overclock their CPUs and then need a way to test how stable they are afterwards. So, for me, prime95 is the wrong tool for the job you're trying to perform (but that's a personal opinion). However, if you're wanting to use it, then you need to run it for at least 3hrs. If your iMac continues to run it without error, then it's passed the test.

If you're particularly concerned about your hardware, I'd start with using Apple Diagnostics to test your machine.

Run Apple Diagnostics

Follow the steps below to run Apple Diagnostics:


Fully shut down your iMac
Restart your iMac
Immediately press the D key and keep it down until you see the Diagnostics screen appear
Wait for Diagnostics to finish (this typically only takes a few minutes)
Once complete, one of two things will appear on the screen:


a No issues found message
a brief description of any errors found plus further instructions

If the diagnostics test does find errors, take a note of what they are


Note: If pressing and holding the D key at Step 3 doesn't work, start again at Step 1 and, at Step 3 press and hold both the OptionD keys instead. This will try and run diagnostics from the internet instead, so you will need to allow more time for it to complete. 

Assuming the iMac passes this without error, I'd be more inclined to test your hardware in Safe Mode to see if you can recreate the issues you're having. If you can't, then you know it's most likely a software issue relating to a login item, corrupted font, or kernel extension.

For more info on Safe Mode, refer to: Use safe mode to isolate issues with your Mac.