I've had the same thing happen on mine. The phone would freeze up so bad that even though it was on, It was so locked up that it didn't even register that i pushed the power button to turn the screen on. After removing a lot of the apps that run in the background all the time I haven't had any problems since. I got rid of my weather app that constantly displays the weather in the notification bar, facebook messenger, and all the battery saving apps i've been trying. You can find the apps that run in the background constantly if you do a reboot and then go into settings, apps, and swipe to the 'running' section you'll see all of the apps that are running and how much memory in MB they are taking up. After removing a lot of offending apps my phone seems to be performing a little better.
I guess the point is, is that the Valet has about 1/6 the processing power and RAM than let's say the Galaxy S5 so it's hard on the phone for there to be tons of apps that it has to run in the background so I suggest that you try removing some apps that show up as running in the background via the tool in settings and see If your problem persists.
People earlier suggested using battery saver apps and cleanup apps but you have to be careful on which ones you use. There are a lot of apps the claim to free up your ram by constantly killing apps that run in the background but they can cause a lot more harm than good. First of all, installing such an app just adds another process that your phone has to be constantly running in the background which to a point is bad for your battery life and overall performance, especially on a low end phone. Secondly, the android operating system will cache recently used apps in any free RAM that is not being used so that app can be loaded more efficiently and a lot quicker when you open it again. However, android will also kill the cached app from RAM as needed If you don't have enough RAM to run the app you are currently launching. Due to the nature of the Android operating system, apps that constantly clear your RAM can actually hurt the performance of your phone by clearing those cached apps that allow those apps to be opened more quickly and efficiently so your phone has to go through all the work of loading it from the disk again. For these reasons I do not encourage anybody to use those kinds of apps.
On the other hand, there are apps like CCleaner that do not constantly run in the background but give you tools to keep your phone's storage. Cleaning up your phone's storage regularly (but not excessively) will possibly fix problems with crashing and improve your phone's performance.
If removing apps that bog down your phone and clearing your caches with a safe tool like CCleaner does not make a difference in your phone's performance and your phone is still constantly locking up, one of the things you can do is either reset your phone to factory settings.
I hope I helped!