Please help me identify my build problem

[Zol Rot]

Recently changed a bunch of components in my computer, mid-high range stuff in there. Yet I'm (rarely, like a couple of times a day for like 10 seconds) experiencing frame rates of as low as 45 fps in Borderlands 2, which is a pretty old game for this setup in my mind. I'm probably overlooking something and I feel super stupid about this, so please help me identify it.

My setup as of right now:
MB: ASUS Z170 PRO GAMING
CPU: Intel Core i7-6700K Skylake
GPU: MSI GeForce GTX 980 Ti
RAM: Corsair Vengeance LPX 2x8GB
CPU cooling: Corsair H100i
PSU: Chieftec A-85 Series CTB-650S 650W

Everything in here is less than a week old except the CPU cooler and my PSU, which are a couple of years old.
I feel like (and hope) it might be my PSU (which I am planning to change withing 2 weeks' time). However, some brief research implies that this should be sufficient, and also that a PSU cannot affect framerates. I don't know a whole bunch about PSUs, but it's still the only thing I can think of that might be the source of my issue. Can it be the fact that it's getting kinda old that is giving me problems? Just worn?

I'm 100% positive everythig is on point as far as drivers etc goes, and I have been monitoring my temperatures closely so I know those are just fine. No overclocking going on, if that is of any interest.

So TLDR:
Is my PSU the issue? I think and hope it is because I'm changing that anyway. Thanks in advance guys

 

[IAmTheTofu]

It might be an unstable overclock. The PSU is not your issue. Just keep it at stock speeds for now by resetting the BIOS settings. If that doesn't work, then it might be something else. Not your PSU.

However, I would get a better PSU before it fries your entire build.

 

[Groveling_Wyrm]

I would say your hard drive is probably the issue.....it keeps caching....

Your GPU and CPU can max out a hard drive pretty easily.

 

[Zol Rot]

It might be an unstable overclock. The PSU is not your issue. Just keep it at stock speeds for now by resetting the BIOS settings. If that doesn't work, then it might be something else. Not your PSU.

However, I would get a better PSU before it fries your entire build.

Like I said, nothing is overclocked. I always keep my stuff stock for the first while to make sure it's running smooth, which in this case it's not Besides I'm aware the PSU isn't ideal, so like I said I'm going to change it within a couple of weeks.

Thanks for taking the time to give me some advice though!

 

[Zol Rot]

I would say your hard drive is probably the issue.....it keeps caching....

Your GPU and CPU can max out a hard drive pretty easily.

Right, I should have probably posted those aswell. I have an SSD (250 GB) with a cache size of 512 MB I believe (this one contains both the game and windows so I guess that's the relevant one), and a regular thingie at 500 GB (cache size 16 MB I think.) for junk. I really only use it for storage purposes. What size would you recommend for this setup?

Thanks for taking the time to give me the advice!

Edit: hadn't mentioned the cache size of my HDDs.