Is my CPU bottlenecking?

It isn't going to make that much of a difference, but Intel chips do have better IPC and that processor is pretty weak, even by AMD standards. On a CPU intensive game, an i5 would help. It isn't going to be such a big help that it would warrant a new motherboard and chip, unless you plan to sell yours and get a decent deal, overall. Remember to bundle the cpu/motherboard, to get a better deal.

Open task manager (CTRL+SHIFT+ESCAPE) and go to the performance tab. Then, go play whatever game you want. When you run into the lag, ALT+TAB out of the game and look at the task manager to see how much CPU you were using when the lag hit.

You have an A8 isnt that an APU not a CPU? If it is you should run your computer in dual graphics mode to get more performance out of it.

I wouldn't go with an i5 - you'll need to change your motherboard as well I believe. Better to go with a new AMD chip, but I don't know which ones are good for gaming, so can't say which to go for.

Your A8-3870k is an APU, meaning it has the GPU built-in. Make sure you have your primary GPU set to your dedicated GPU.

You cannot simply "upgrade" to an i5 - you need a new motherboard as well, and possibly a new PSU depending on your output wattage and amperage.

How much RAM do you have?

Here's a pic of the CPU and GPU usage

why spend 200$ on a GPU but still get a 100$ APU?
you should have gone with amd fx or intel i3-i5 (whatever budget you can set)
now you cant upgrade unless you change the motherboard and CPU, and that would cost +200$ to be considered an upgrade

also try to go back to stock CPU speed, if the frames drop then its a CPU bottleneck

the best graphics card you can buy without the AMD A8-3870K limiting performance in games is between the AMD Radeon HD 7850 for a 1080p monitor

the AMD A8-3870K and the AMD Radeon HD 7850 2 GB are well balanced. if upgrading to a better graphics card it would be neccessary to upgrade your cpu to avoid a cpu bottleneck. Red Orchestra 2 is a graphics demanding game and lowering the settings will improve performance

As for AMD dual graphics i would disable it because the AMD Radeon HD 7850 does not pair well with the APU graphics. Enablng AMD dual graphics will negatively impact performance due to the drastic difference in performance between the AMD Radeon HD 7850 and APU graphics. Also disabling AMD dual graphics should increase cpu performance because the AMD A8-3870K will be solely dedicated to cpu performace instead of both cpu and gpu performance

i5

Yes it would be worth to spend this money on i5. You should buy cheapest i5 (3350P) and quite cheap motherboard (for example MSI B75A-G43). A8 isn't good CPU - even Phenom II is much better. If you would overclock i5, you must buy 2500K/3570K and motherboard with Z77 chipset. Moreover you can wait for new Haswell i5, but they can be expensive.