T420s Multiple Monitor Issues

[jamiphar]

Wow, it's been a while since I've been here!

My trusty T420s has had an issue with my monitor setup and I've finally decided to do something about it. I have it on a Mini Dock 3 and use three external monitors connected to it. Monitor 1 is 1920x1080 and connects via DVI, monitor 2 is 1280x1024 and connects via VGA, and monitor 3 is 1280x1024 and connects with a Displayport to VGA adapter.

This setup was working fine for a long time. I could use all three displays and even my laptop screen as a fourth simultaneously. At some point, I lost some functionality and I can now only have two external monitors working at a time. If I have two running and connect the third, the displays keep refreshing every five seconds or so and the additional monitor appears and disappears endlessly in the display settings. I can use two external monitors while using the internal display, though.

I've tried updating video drivers, reverting to a factory installation, making sure Optimus is enabled in the BIOS, etc., and I haven't been able to solve it. All of the ports, monitors, and cables work fine when tested individually, but I can't use more than two monitors at once. I've seen discussions about a similar issue on newer ThinkPads and docks, but I couldn't find too much on the T420s-era machines.

Any ideas?

[brchan]

Set the graphics mode to discrete in the bios. The system might only be using the intel gpu, which supports fewer monitors than the nvidia gpu.

[jamiphar]

I gave that a try and it simply limits me to two monitors. Choosing discrete mode disables the Intel GPU.

[jondoh]

have you changed power supplies recently? i know on some brands, if you don't have a powerful enough supply it can affect weird things.

[jamiphar]

No, I believe I've used the same power supply the whole time. I did test a 135W supply with it just before I posted, and I didn't see any change.