Graphics trouble after unlocking Windows

I'm currently experiencing a frustrating problem and have not been able to find much info on it so far. In fact, I have only found a single case similar to mine anywhere online; graphics trouble after unlocking. I work for a company and have deployed around 25 workstations with the following basic specs:

Dell OptiPlex 9020
Windows 8.1
AMD Radeon R5 240
Dual Dell P2214H Monitors connected via DisplayPort 

The issues started straight out of the box.

Occasionally when a user unlocks their computer after leaving it idle for several minutes, one monitor will be messed up. This could either be a completely scrambled image, as described in the link above, or the resolution will be locked at 640x480. Disabling and enabling the driver may fix the problem temporarily, but the it will eventually come back. It has not happened on every computer and does not seem to be related to any specific power setting. (All workstations have hibernate and sleep disabled, but we allow the users to select how long to wait to turn off the display.)

We have tried swapping out DisplayPort cables, switching to the DVI port, and updating graphics drivers. All seem to have a slight effect, but nothing has proven to be a complete solution. Any suggestions or advice would be greatly appreciated and I'll be happy to provide any additional details as necessary.

I'll make sure update this post if we manage to find a fully stable configuration. 

Thanks!