I just been doing some testing and my idle temps when clocked to 5ghz is at 30-35 and the prime5 test is 70-80 just wondering should i underclock it or is this a safe temp to game at?
The problem is the 9900K is difficult to keep cool consistently. Frankly, I would see what your temps are under the normal load you would expect to put it under.
Prime95 is a great gauge, but 80 degrees is a bit too hot, and won't be great for maximising the lifespan on the CPU. But ultimately, I don't expect many applications to stress the CPU as much as Prime95, so you're probably fine.
Way to find out is to monitor temps under what is regular load for your usage.
So to put it briefly - 80 degrees for me is too hot to be gaming at regularly, but Prime95 will stress the CPU more than many games will.
Core temperatures below 80 degreees should be fine generally. But be wary if you're going over that.
Obviously if you've auto overclocked, just make sure it's not pumping unnecessary voltage in which in turn can cause more heat.
But as long as you stick to below 80 under load, you'll generally be fine, anything pushing above 80 and you'll need to think about improving cooling.