Whoever is still using r9 280x... I need your advice badly!
I have weird 13-26 mh/s ETH 29-58 mh/s LBC fluctuation.
Does not depend on overcloack, power limit, page file > 32gb different PSUs voltage, temperature or whatever, tried all the shit possible!
It`s like >2 times difference! And usually it is ~13...
In your case, Nicehash switches among different Ethash coins - Ethereum (DAG ~145 epoch), Ethereum Classic (~148 epoch), Ubiq (~8), Expanse (~23) and Musicoin (don't remember, about 15-30). AMD Tahiti GPUs are very sensible to DAG epoch, i.e. the DAG size in GB and with each epoch, the hashrate of Tahiti chips drops about 0.15 MH/s (estimate taken from forums).
AMD Polaris, RX4xx and RX5xx, did also exhibit hashrate drop connected to increasing DAG file, but the AMD Blockchain Workload beta driver fixed that issue. AMD will never release a similar software patch for AMD 7xxx, 2xx - these products are phased out.
I advise you to run a dedicated Claymore miner instance on these 280x, mining Ubiq, Expanse or Musicoin at a decent rate of 24-27 MH/s, rather than switching infinite times per day between ETC/ETH at 13 MH/s and UBQ/EXP/MUSIC at 27 MH/s.