I have the same problem but with a nVidia card:
sudo lshw -c video
*-display
description: VGA compatible controller
product: GT218 [NVS 3100M]
vendor: NVIDIA Corporation
physical id: 0
bus info: pci@:01:00.0
version: a2
width: 64 bits
clock: 33MHz
capabilities: pm msi pciexpress vga_controller bus_master cap_list rom
configuration: driver=nouveau latency=0
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https://bugs.launchpad.net/bugs/1079440
Title:
Random complete freeze using dual monitors
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