MSI Afterburner is probably the best overclocking tool on the market. Just curious, what sort of temps are you getting when stressing your 7870? Also, since the card already comes at 1100MHz, either you've gotten a dud that won't go much higher then 1100Mhz in which case really sucks or, you simply have to increase the voltage because you're card might not be stable enough at the speeds you are running.
Tempature with the fan just at 30 percent GPU AT 1250MHZ and the Memory Clock Around 1350 the card runs around 40-42°c
I am talking about when just putting it to the catalyst max allowed of 1400/1450 I have had some crashes