I've been using target display with my iMac for several months - output from a PC through HDMI port to the mini-displayport on the other end of the cable...
Anyway, no problems until this week... Now suddenly when I enable the PC (either by plugging in the cable, or command+F2, which is the correct shortcut to switch displays) it switches over for a few seconds then just switches back without my intervention. I also noticed that it sometimes does some flickery things (screen flicks, shows some garbage for a few seconds) here and there while it's switching over, or occasionally refuses to switch over at all...
Seems strange that it worked fine for so long then just suddenly stopped. Anybody had the same thing happen? Any ideas for a fix?
I tested the PC in to another monitor and it worked fine.
(One thing I noticed, which may or may not be related, is that occasionally when plugging the mini-displayport in to the iMac it would make a tiny blue spark or 2... So small they were almost unnoticeable, and it did this many times without incident when I plugged/unplugged stuff... But maybe was some kind of damage being done...?
System details: PC is running dual GTX-670 in SLI mode on Windows 7 (normally set at 2560x1440 @ 60Hz) Mac is 27" model running 10.8.2
