it says invalid checksum and then lost connection to server. is that because the pool is down?

Did you undervolt and / or overclock your cards? try --shaders 1792

Why don't you post what your QQMINER command line here.
BTW, CGminer doesn't work, you need QQMINER.

try:
-I 13 -g 2 --thread-concurrency 8192
or
-I 13 -g 1 --thread-concurrency 8192
or
-I 13 -g 2 --thread-concurrency 4096
or
……
You can finger it out.

Just try:
-I 13
or
-I 12

try lower intensity, from 20 to 19 for example

Thouse are the best settings for me
R9 280x :
cgminer.exe --scrypt -o stratum+tcp://POOL -u USER.Worker -p PASS -v 1 -w 256 -I 13 -g 2 --lookup-gap 2 --gpu-powertune -20,-20 --gpu-fan 59,50 --temp-overheat 85,85 --temp-cutoff 90,90 --gpu-memclock 1500,1500 --gpu-engine 1000,1000 --thread-concurrency 8193
341kh/s

R9 290 :
cgminer.exe --scrypt -o stratum+tcp://POOL -u User.Worker -p PASS -I 19 -g 1 -w 256 --lookup-gap 2 --thread-concurrency 24550 --gpu-engine 1000 --gpu-memclock 1500 --gpu-powertune 10 -v 1 --auto-fan --temp-cutoff 85 --temp-overheat 83 --temp-target 78

447 kh/s

If i helped someone out if u want to tip : VfUzaU29Ri4oFGhBPLHsmsqMqh97uZ4wbS

WTS 100 pot for 0.01 btc