The problem is the second 16x Slot, my current miners works with one 16x to 16x risers and three more cards with 1x to 16x POWERED risers.
As long as you don't use the second 16x slot (really 4x, but doesn't matter for mining is the same 1x than 16x) you are good,specially in linux.
the throttling you are seeing, that goes back to 400Kh is probably the powertune, set powertune to 20 and try again, also, if you have some monitor program like GPU-Z or TRIXX, shut it down, the pooling generated by those programs slows the card down.
I don't think it is a driver problem (the code 43 thing) but you can try, load TRIXX and click DISABLE ULPS, and click load on windows startup, and restart, see if you can detect the card with that (I don't think so, I am pretty sure it is the motherboard's second 16x slot, but it is something easy to try!.
comment results plz.
cya
Indkt
Finally got it what was wrong... second GPU was defective.. :-(.. sent back for a new one... should arrive tomorrow.. as for the hashrate, got it stable 705kh\s. it was always dropping to about 450khs because i was mining quark with the CPU and i only have 4GB of RAM.. as soon i turned the Quark miner off.. boom!!! 700khs.. i managed 750khs but was not getting accepted shares.
Also with 750khs as this GPU is a MSI r9 280x it has some bios issues.. and bootting from the usb qith this mobo to update the damm thing is really hard..
I will keep the 705khs..
here are my setting for 705khs stable..
setx GPU_MAX_ALLOC_PERCENT 100
setx GPU_USE_SYNC_OBJECTS 1
cgminer --scrypt -o stratum+tcp://mine1.coinmine.pl:6300 -u gleydson.1 -p x --lookup-gap 2 --thread-concurrency 8192 -g 2 -I 13 -w 256 --auto-fan --gpu-fan 30-75 --temp-cutoff 90 --temp-overheat 85 --temp-target 72 --gpu-memclock 1500 --gpu-engine 1020 --gpu-powertune -20 --expiry 1 --scan-time 1 --queue 0 --no-submit-stale
thanks