Have you optimized your catalog recently?
Furthermore, when Lightroom starts reloading the fields of Metadata panel, all the letters that I was typing in Title/Caption fields become the commands that I give to Lightroom!
This suggests that the LR user interface is going wonky, and the keyboard focus is shifting away from the Metadata panel for some reason. I suggest three troubleshooting steps:
1. Reset LR's preferences: http://www.lightroomforums.net/showthread.php?25197-Resetting-the-lightroom-preferences-fi le-updated.This often fixes things when LR's user interface goes wierd.
2. Do you have any third-party plugins installed? If so, temporarily disable them via File > Plug-in Manager. Scroll to each plug-in, then in Status on the right, click Disable.
3. Download the free trial of LR CC 2015 / 6 and see if the problem occurs with that. (Make sure you're starting with the reset preferences from step 1, since LR upgrades will use the preferences of previous versions.) The free trial will convert your 5.7 catalog without modifying it, so you can freely experiment with the 2015 / 6 catalog. Note that if you decide to go back to 5.7, any changes you've made will in general be lost, so don't do any permanent work in the 2015 / 6 catalog until you've decided you're definitely upgrading.