I have Ryzen 5 3600(Stock Cooler), Aorus B450 Motherboard with Geforce GTX 1660 GPU and PSU of 550W. My CPU temperature is constantly in 70-85degree C while using only 11% of usage of CPU at 4000MHz. GPU temp is about 40-60degree C.
Reapply thermal paste and make sure that the cooler is mounted properly...Make sure you connected the fan to the motherboard.
If,after all this,the temps stay the same,you can install a monitoring program ( Ryzen Master or MSI Afterburner,CPU/GPU-Z etc.) and see what's exactly happening when the system idles and gets stressed.(Voltages,Fans RPMs etc.)
If the Voltages are stuck at 1.4+,which happened to me before,you can go into the BIOS and load the default settings or reset CMOS...If this don't fix the issue you can update the BIOS.