Performance problems with FX8350 and Radeon HD7870

There are a few things we can try to nail down your issue.

first i want to eliminate temperature from the picture. so open the side of your case and stick a room fan in the opening, put it on high and try those games again. see if the FPS drops at all.

If the problem vanishes then we know your problem is 100% temp related. and i'll give you some suggestion on how to fix it. If the problem doesn't go away i can suggest a few other things to try including walking you through overclocking that cpu. Let us know how it goes with the fan first.

i don't trust any temp reporting until i know it's reporting accurately. AMD cpus are notorious for inaccurately reporting their core temps. Furthermore of all the parts to not work well on a motherboard, the onboard thermometers are on the top of the list. Lets just "make sure" the temps reported are accurate and you don't have any temp throttling going on. once that variable is removed we can try to find software/hardware fixes.

mmm... well you are using a 8350. If you plan to overclock i'd say go for broke and get yourself an h100i. grab some gentle typhoon AP-15 fans to replace the stock corsair ones, and go to town. If you don't plan to blow the doors off your house, you can go with something lower on the totem poll. Something like an h80i or nh-d14. Of course your motherboard isn't what i'd call a great overclocking board, so just swapping over to a hyper 212 evo would probably be a step in the right direction.

As for your immediate problems, we're still not sure what's temp throttling, your motherboard or cpu. Your motherboard lacks mosfet heatsinks. With an 8 core piledriver this can be a problem. So first what you'll need to do is ~

1) get yourself 2 80mm fans, and some double sided tape. tape them end to end, and then tape them to the back of the io panel blowing down on the mosfet along the back of the cpu. See if this removes your temp throttling with the case closed. If it does, then your problem was the lack of mosfet cooling, not the cpu temps. In which case i would order some mosfet heatsinks like these and put them on your mosfets.
http://www.newegg.com/Product/Product.aspx?Item=N82E16835708012 (this is not an endorsement of this particular product, i've never used this product, but if it was my board this is what i would probably buy to fix this issue)

2) Check the thermal paste on your cpu cooler. take the heatsink off the cpu. Visually inspect the lid of the cpu. see if any thermal paste is running over the sides, or if the thermal paste doesn't cover the lid of the cpu. If either happened you might have too much/too little paste. clean both the cpu and cpu cooler with rubbing alchohal and qtips. Let them dry. make sure both surfaces are shiny and clean. Apply a bead sized blob of thermal paste in the middle of the cpu. press the cpu cooler down on top of it WITHOUT rocking the cooler, or picking it up again. both will put airbubbles in the paste and ruin it's functionality. twist the cooler a little back and forth to spread the paste. secure it. test it.

3) if the paste was applied right, and it still throttles, try aiding the airflow within the case with a 120mm case fan secured with dental floss or double sided tape. make sure it's placed somewhere between the front fans and angled toward the cpu cooler. the idea is to keep the airflow moving front to back, bottom to top through the case.

If these three things don't work you might need to look into modifying the case to improve airflow, or experimenting with additional fans in additional places to keep the air moving right inside the case, or working on cable management to unclutter airflow, or getting different and better case fans (like those aforementioned gentle typhoon ap-15s). there are lots of small tinkering you can do to lower the temps in that case. and improve performance. You might have to take my suggestions and come up with your own unique solutions to solve this problem. Thats part of the fun of working on these computers.