I remember having the same lines in syslog with a quite antique ATI Card (X1950 if I remember correctly) with the same driver. After some time GPU locked up, screen turned off and then all the system became unresposive (couldn't even ssh to it). There was no way to reliably reproduce this issue. In my case the problem was not a software one, but rather a hardware -- I fixed it with replacing PSU, no problems after that.
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