which r9 290 do you have and what settings do you use for them?
It's an XFX I just picked up last week. Cards max out at --shaders 1792,2560 --shaders-mul 9,6. Got three New block detected's so far and four Found Block's. Four hardware errors on the 290, none on the other.
Could be card/driver/bios. I just picked it up and haven't thrashed it into shape yet (damn, no hynix mem!) I think the person who sold it to me thought they were dumping a lemon. It had one of those awful blocks for a cooler and my jaw dropped when I saw it was idling at 80C! But I intended to slap this Thermaltake 2.0 on it anyways, and now it stays under 70 no matter what the voltage or load. Quite possible he might have cooked it a bit just trying to run anything.
So, one valid block somewhere because I'm at 5613 now. Going to update those drivers, back in a bit.