My 2 * 280x both are stable for Zcash mining using Claymore's ZCash AMD GPU Miner v8.0, when i adds 2*290x my rig gets hang/black screen or sometimes system shutdown.using following code in two different start bat file also tried in configure file;
for 280x
ZecMiner64.exe -zpool eu1-zcash.flypool.org:3333 -zwal MY WALLET ADDRESS -zpsw x -mport 0 -i 5 -di 01
for 290x
ZecMiner64.exe -zpool eu1-zcash.flypool.org:3333 -zwal MY WALLET ADDRESS -zpsw x -mport 0 -i 5 -di 23

also tried this;
for 280x
ZecMiner64.exe -zpool eu1-zcash.flypool.org:3333 -zwal MY WALLET ADDRESS -zpsw x -mport 2345 -i 5 -di 01
for 290x
ZecMiner64.exe -zpool eu1-zcash.flypool.org:3333 -zwal MY WALLET ADDRESS -zpsw x -mport 0 -i 5 -di 23

also tried -mport as 3333, -3333, (-mport 0, -mport 2345), (-mport 3333, -mport 0).

problem facing when claymore's v5 was released.