This sounds like an issue related to the drivers on the phone interacting with the hardware. In laymen's terms, The software on your phone is having issues talking to the hardware. I would suggest taking your phone in and having the techs at sprint flash your phone back to factory specs. Any contacts that are on your phone and any apps would be gone... However since its an android phone the Gmail account associated with your phone will restore those contacts, and any twitter/Facebook/outlook contacts would also be restored when you add those accounts back into the phone. Your pictures should be stored your SD-card and as long as it was removed before the flash those pics would be saved. If this does not help the issue then it may be related to the hardware its self and would require a new device
Sadly I've had this issue and more with my phone. I ended up taking it into a service center (on multiple occasions) and they had to do a hard reset. They couldn't explain the exact cause or even how to prevent it. Unfortunately it appears the only people really in the loop about the HTC Evo is HTC themselves. I'm also waiting for my upgrade to dump this thing. I've had more system restarts and resets than I care to count.
I wanted to add that a factory reset can be done by holding the vol down + Power button at the same time, you can move between options to highlight 'Clear Storage', and hit power to select it.
Alternatively, you may want to root it and put a ROM on it, because this sounds like it may be due to a faulty ROM installation (Basically, their OTA).
They aren't really made to last very long. My evo 4g was insured when it crapped out and the insurance company replaced it with an evo 3d which has much better specs. I should have taken it to the store first though because i had to pay the insurance fee of $100. Take it to the sprint store and see if they can fix/replace it.