Single character display corruption with radeon driver

Using the open source equivalent of fglrx, ie using the radeon driver at /lib/modules/3.13.0-24-generic/kernel/drivers/gpu/drm/radeon/radeon.ko, 2 or 3 months ago, I got many instances (ie occasionally 1 per screen, but several per day) of single characters displayed using a corrupt glyph. Sometimes the glyph looked like a mixture of 2 characters. It was an AMD Radeon R7 260X. The driver module appears to be provided by package linux-image-extra-3.13.0-37.64 now, it was 3.13.0 -24 at the time. I had no such problem with the fglrx closed source driver. I was using cinnamon.

Here are some examples of text that had 1 char display corrution, last char shown is the one that was corrupt:Version: 4.1.1
forums.linuxm
forums.linuxmint.com [ie both of last 2 m's corrupt]
URL : http://
Are there any usage restricti
User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_6
Server
Wireshark says "There
Wireshark says "There are
Wireshark says "There are no inte [ie all of last 3 e's corrupt]
Jul 30, 2014 ... Platform-Specific
amd64-microcode not install
    arm64: Tell irq work about self IPI
Bull
GPG
AndEngine - Forums • Pixel Pe
Connection to
Radeon OSS
Device drivers and
Nvidi
http://
http://www.mplayer2.org/
alca
ATI/A
documente
Server
Overv
Overv
Overv
Overv [possibly easier to repeat this (in firefox) by changing to a tab that hasn't been used recently, then back]
 - https://github.com/TresysTechnology/refpolicy/wiki
User-Ag
/usr/shar
/usr/share/initr [both r's]

Maybe it is assuming recently rendered char is still in a cache, when it isn't - larger char might restrict the repetition to be
more recent.

The only place where it was repeatable every time was in synaptic->settings->repositories->Additional repositories where one of the entries always contained the corruption the first time the additional repositories button was selected after opening the window. It may have been getdeb. So it was not necessary to restart synaptic to test it.

1. It was Mint 17.
2. Install an R7 260X and the radeon driver, and use cinnamon all day, or use the synaptic option as above.
3. Occasionally 1 character was displayed wrong (using copy-paste on it still got the correct character).
4. The characters should have displayed using their normal glyphs.
5. Sometimes (except always if given enough time), or always in the synaptic case.