r9 280 and r9 290 invalid share all time.
solution?
If you are solo mining then you wont get anything. If pool mining then you might get something as those cards don't seem to work well with the miner. Just 8+ Mh/s. R9 290 ones are killer cards and should have been doing way better than Rx series.
I'm mining in suprnova and with r9 390 works at 17 mhs but r9 280 and r9 290 only got shares invalid.