The cores are NOT bad. The mobo settings are less than perfect to run all 8-cores successfully under the P95 stress test. All cores are not exactly the same so a couple might need some fine tuning to run P95 without issue. A couple weaker cores are common. Minor BIOS tweaks will usually fix the issue if you have:
1. An FX-8000 series CPU approved mobo
2. Proper cooling of your CPU (core temp below 61C under load) - VERY IMPORATNT
3. A PSU and mobo design that can supply the power required
4. Your RAM passes an overnight test of Memtest86+ V4.20 or V5
Your Asus mobo should support the FX-8350 and as long as the other items are good, I'd spend a little time working the BIOS settings.
Often the power saving features can cause issues during P95 stress testing which is extremely tough for an 8-core CPU. The easiest way to check these is to disable Cool & Quiet, C6 mode, CPU thermal throttle, Turbo core tech, Application Power Management and C1E. Asus may use slightly different names but these are the key power saving controls.
If P95 runs OK with these disabled then you can step by step turn one item on until you find the issue. I've also found that raising the RAM voltage by .05v helps stability with 8-core FX processors. There are a list of other BIOS changes that can be made if you desire to run in manual mode but you might want to try the above suggestions first.
Building a reliable PC involves more than just assembling the parts. You need to be able to configure all of the BIOS settings appropriately. This can be quite involved and frustrating as it can require a lot of trial and error with stress testing. It is however often the only means to get a 100% reliable PC.