This usually only happens if you have rooted the device, which most people do when trying to get a version of Android on the device that their device is not able to get through updates. Which in most cases also is not really capable of running well on said device.
If you have not done that, then I would be looking at what you have installed recently. You can try starting up the phone in "Safe Mode" and see if the problem goes away in there. Most apps won't run in there (as it is a restricted mode) but if it doesn't happen in this mode then you know there is something on the the device that needs to be removed. Bad app, virus, malware, etc.
As a last resort you can try backing up your data and doing a "Hard Reset" of the device. However, if the device was rooted (as mentioned above) you can't do the reset until it is unrooted.
To get into "Safe Mode" please do the following.
1. First turn off the phone.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.
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