Built PC i5 4690k running 100 deg. c directly after boot, and down to 50c idle, and 80-95 during gaming, and then stays at 100 stress test.?

Just as a comparison, download RealTemp from the internet (it's free). At idle I run at about 25 degrees, with lots of non-gaming apps I run at up to 45 degrees depending how much is running at any one time. When gaming or video processing I can run at up to 70 degrees. Never higher.

If you're running much higher temps using realtemp then you have serious problems and you're in danger of blowing your CPU or motherboard. You need to do something soon.

First of all, take off the heatsink and fan to the CPU. Clean them of all dust and paste with compressed air. Next apply a good thermal paste (I use Arctic MX-2) - you can use the pea method or the credit card method. I swap between them and rarely see much difference of more than 1 or 2 degrees.

Next clean all other fans and heatsinks in the computer. If you're still having problems you have a dodgy CPU or dodgy temperature sensors.

You could always consider watercooling - it's expensive but definitely worth it. Even if you upgrade later you can swap the watercooing equipment over to your next computer.

^ This seems very hot for a processor, fan is on properly, only a month old, dust-free, no viruses on the PC, so is it normal to be this hot?

ALSO.. there's NO sign of any performance issues. Games run perfectly with a gtx 960 and the case has great air flow. Also the graphics card runs very very cool, almost always half the heat as the processor.

I would re-paste it. This is not normal. It's also bad for you because it's obviously throttling the processor.

Turn off speed-step (turbo boost) in BIOS if you want it to run cooler.

Wow, that's too hot. Did you apply the right amount of thermal paste? Did you make sure that the heat sink is mounted correctly and that the airflow in the case is not significantly obstructed?