I explained to the store that these cards were all burning even though held at optimum conditions. They apparently tested them and they sent me today 3 replacements but with Sapphire R9 280 instead of HD 7950.  (Not the 280X version!).  i think it is essentially the same card but just a different series.

I still have another 3 burned 7950 cards to send back, but they said that it would be ok to replace those too with R9 280.

The fact is that the HD 7950s fire up at a whopping 300 W against 220 W for the R9 280, while having a slightly higher hashrate.  So I'm happy to exchange them.