Originally Posted by dae
LI upgraded the drivers from 260.19.44 to 270.30, and that appeared to have solved the problem. The monitor connected over the DP->DVI dongle is now suspending correctly.
I'll have to take that back. It did work a couple of times when I tried it manually. Now it's back to the old behavior, cycling between on and off. This happens both when manually activating dpms, and when waiting for the normal timeout to occur.
I really have no idea why it stopped working; I haven't changed anything, and I haven't restarted neither the computer nor X.
Let me know if there's any more info you need.