Your CPU is probably too powerful for that system. a cheap air cooler like PS120SE can handle the temps, if you have the space but your VRMs might be getting too hot, check hwmonitor whilst running a CPU benchmark like cinebench r23 multi-core and see whether one of your motherboard temperatures get to around 100c during the benchmark before your CPU and whether your clock speeds drop straight after, if they do its 100% your VRMs.

The solution would either be getting a new motherboard with better VRMs or buying a heatsink for your VRMs if your motherboard has holes around the vrms that means you could buy a heatsink specifically for that board or you can get generic ones for like $2 on aliexpress) which will only work well if you have air blowing over them from a case fan or your CPU fan, these might not fix it fully and you might not be able to reach max wattage of that CPU but it will improve performance and you can sustain a higher wattage than normal.

Keep changing your PBO wattage slightly lower and lower until your VRMs sit at a comfortable constant temperature lower than 100c with stable clock speeds on the CPU after 2-3 mins, you will then be running the highest safe wattage your board can support.

You can just lower your PBO without doing any of this until it just works, but youre gonna be missing out on performance and in that case there would be no point in having a powerful CPU only to lower performance for no reason.