S4 Constantly Lags/Freezes randomly

Hello,

My Galaxy S4 phone tends for freeze up/lag a lot in the last month.

It lags when:

-My phone is locked and I hit the power or home button. The screen quickly flashes on and off, and will stay dark until a minute or two later when it finally releases itself and then I can use the phone again.

-When I'm texting. The screen freezes and can't type anything. After a few moments the screen goes black. I hit the power/home button and everything works again.

-I'll be randomly using the phone, and then the phone freezes and like the previous post, have to wait til the freeze/lags releases the phone before I can use it again.

Note:

- The freezing/lagging happens randomly. It can happen after 6 hours of usage, or after 5 minutes after powering up my phone.

-Did a factory reset, but that hasn't solved the problem

-Wiped the Cache Partition. Nothing.

Is there a way to isolate the problem and figure out what is causing this?

Thanks!