Hi,
The .plists are in the ~/Library (and then a sub folder) so could be described as a "Library" file.
Having been messing with iChat and now Messages for the last 15 years I have come to know which files that can be deleted on their own to have the most effect with the least destructive effective cause by their removal.
That said there can be thousands of .plist in the Preferences alone.
There are many for files in ~/Library/Containers as there are nearly 1 for 1 sub folders for every Apple App for the Sandboxing process that protects one app from bring down the whole computer when it has an issue.
Do you use Time Machine ?
And do you know the date this started ?
IF so try this:-
Open the Finder's window in to the ~/Library (the bit where you used the ALT Key to view the change Menu list and selected Library.
Highlight the Preferences so it is sort of pinned to this point.
Now use the Time Machine icon in the Dock.
This will change to the galaxy background and possibly a smaller window that looks stacked above others.
On the right are dates and arrows (the dates will be each update that has been made)
If you use the arrows to go back in time it will scroll though the windows to the last update of the Library.
Go back to a date before the issue arose.
Then use the Restore button whilst still selecting the Preferences Folder.
It will move to the first window and bring forward that older file to the "present".
When this is done Quit Time Machine.
Restart the whole computer.
8:50 pm Monday; June 1, 2015
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad