Well... I just cannot get my GTX 280 to go above 16 khash/s and would really appreciate some help considering I should get more out of it based on the GTX 260 results.
Autotuning doesn't come up with anything. When I use -D, it shows:
D:\Litecoin\cudaminer>cudaminer -D -o stratum+tcp://netcodepool.org:3333 -O user:pass
[2013-11-29 21:49:15] Starting Stratum on stratum+tcp://netcodepool.org:3333
[2013-11-29 21:49:15] 1 miner threads started, using 'scrypt' algorithm.
[2013-11-29 21:49:15] Binding thread 0 to cpu 0
[2013-11-29 21:49:15] Failed to get Stratum session id
[2013-11-29 21:49:15] Stratum difficulty set to 64
[2013-11-29 21:49:15] DEBUG: job_id='6d14' extranonce2=00000000 ntime=5298fdb3
[2013-11-29 21:49:15] Stratum detected new block
[2013-11-29 21:49:16] GPU #0: GeForce GTX 280 with compute capability 1.3
[2013-11-29 21:49:16] GPU #0: interactive: 1, tex-cache: 0 , single-alloc: 0
[2013-11-29 21:49:17] GPU #0: Performing auto-tuning (Patience...)
[2013-11-29 21:49:17] x1 x2 x3
[2013-11-29 21:49:17] 1: | | kH/s
[2013-11-29 21:49:17] 2: | | kH/s
[2013-11-29 21:49:17] 232: | | kH/s
[2013-11-29 21:49:17] 233: | | kH/s
[2013-11-29 21:49:17] GPU #0: 0.00 khash/s with configuration L0x0
[2013-11-29 21:49:17] GPU #0: using launch configuration L0x0
And then cudaminer crashes.
And I tried some manual settings like
-l 27x3
-l 54x3
But the end result is always the same; "result does not validate on CPU!"
What does work is
-l F27x3
-l K54x3
But the rate does not go above 16 khash/s
And I really do not know what kernel to use.. S, L, K, F,..?
The known GTX 260 setting
-l S27x3
Will then show: "using launch configuration K27x3" with the same (slow) result.
Any help is appreciated! 
