Case fans spinning slow/stopping

Hi,
I bought a Corsair 650D case about 6 months ago (Link to item on Newegg) and have been very happy with it.

Short Version:
-never had any issue until (or at least noticed until) yesterday.
-When i leave my fan controller set to low (as i have the whole time I've owned the case), the top and rear exhaust fans seem to run slowly and sometimes stop. When they are spinning, they push very little air (far less than they used to).
-If i set the controller to high, they will start spinning again.

Long Version:
-Yesterday i noticed that the exhaust fan at the top of the case (200mm i believe) wasn't spinning (computer was on, mostly idle). I almost always keep the case fan controller set to low, and have never had a problem with the fans before. So when i noticed the fan had stopped, I flipped the fan controller switch to high and it started up again.
-This morning I noticed the same thing had happened with both the top and rear exhaust fans. Again, turning it to high got them spinning again.

My first guess was that the fans were crapping out or something, but that seems strange to me for a couple reasons:
-6 months seems like a very short lifespan
-Both fans started acting strange at the same time. If it was just one, I'd say I got unlucky with a defective part or something...
-I'm pretty anal about keeping the case clean. I clean the dust screens often and the inside of the case is very clean. It doesnt seem like theres much that could have "gunked" up the motors or something like that.

My second guess is that somehow the fans arent getting enough power or something like that... but they had been working great for 6 months, and I havent changed anything in the case for months.

Does anyone have any thoughts or advice? Let me know if you have any questions, etc.

Thanks!

Edit: Here is my power supply in case anyone is wondering:
KINGWIN LZ-1000 1000W Modular 80 PLUS BRONZE Certified