I have a hd7970 and hd7950.

1) How old is your GPU? - 3 months old
2) Have you ever OC'd or declocked it? - yes to recommended scrypt overclocks.
3) What intensity settings have you run it at on other coins and for how long? - i13 (cgminer) for most scrypt, i20 for copperlark (reaper miner). I dont mine full time, probably 3-4 days in one hit has been my longest non stop mine.
4) Have you ever over/under volted it? No.

Crashes with Prime Reaper on any sievesize setting after so long, sometime instant and sometime after a few mins depending on the setting. If I remove the sievesize setting and sievesize percentage entirely from the config file reaper sometimes runs, but always produces 0 fermats, and a value for gandalfs that declines over time. The longest it has run without crashing in this mode is around 2 hrs, eventually it crashed too. I have pretty much given up attempting to get it to work now.