Yellow pattern all over application windows
It seems to be a GPU problem.

Apple introduced Repair Extension program for some iMacs sold between May 2011 and October 2012. It is not on main Repair programs site though.

The affected iMacs contain serial numbers with the last four characters of DHJQ, DHJW, DL8Q, DNGH, DNJ9, DMW8, DPM1, DPM2, DPNV, DNY0, DRVP, DY6F, or F610. Source

Check with this numbers and Apple to get a repair.