It seems that litecoin/scrypt support in cgminer is unstable as I've found a lot of mentions of this error in various forums and no real solutions. Some seem to be able to resolve the problem by experimenting with different versions and combinations of drivers and AMD SDK. So far I've tried much and always the same problem. Here's the output from command line:
cgminer --benchmark --scrypt --debug --verbose
[2013-06-07 00:34:54] Started cgminer 3.2.0
[2013-06-07 00:34:54] CL Platform 0 vendor: Advanced Micro Devices, Inc.
[2013-06-07 00:34:54] CL Platform 0 name: AMD Accelerated Parallel Processing
[2013-06-07 00:34:54] CL Platform 0 version: OpenCL 1.2 AMD-APP (1124.2)
[2013-06-07 00:34:54] Platform 0 devices: 1
[2013-06-07 00:34:54] 0 Tahiti
[2013-06-07 00:34:54] GPU 0 iAdapterIndex 0 strUDID PCI_VEN_1002&DEV_6798&SUBSYS_254D1458&REV_00_4&176F368&0&0018A iBusNumber 2 iDeviceNumber 0 iFunctionNumber 0 iVendorID 1002 strAdapterName AMD Radeon HD 7900 Series
[2013-06-07 00:34:54] GPU 0 AMD Radeon HD 7900 Series hardware monitoring enabled
[2013-06-07 00:34:54] Pushing ping to thread 0
[2013-06-07 00:34:54] Init GPU thread 0 GPU 0 virtual GPU 0
[2013-06-07 00:34:54] Popping work to stage thread
[2013-06-07 00:34:54] CL Platform vendor: Advanced Micro Devices, Inc.
[2013-06-07 00:34:54] CL Platform name: AMD Accelerated Parallel Processing
[2013-06-07 00:34:54] CL Platform version: OpenCL 1.2 AMD-APP (1124.2)
[2013-06-07 00:34:54] List of devices:
[2013-06-07 00:34:54] 0 Tahiti
[2013-06-07 00:34:54] Selected 0: Tahiti
[2013-06-07 00:34:54] Preferred vector width reported 1
[2013-06-07 00:34:54] Max work group size reported 256
[2013-06-07 00:34:54] Max mem alloc size is 2951741440
[2013-06-07 00:34:54] Selecting scrypt kernel
[2013-06-07 00:34:54] GPU 0: selecting lookup gap of 2
[2013-06-07 00:34:54] GPU 0: selecting thread concurrency of 22400
[2013-06-07 00:34:54] Loaded binary image scrypt130511Tahitiglg2tc22400w256l4.bin
[2013-06-07 00:34:54] Initialising kernel scrypt130511.cl with bitalign, 1 vectors and worksize 256
[2013-06-07 00:34:54] Creating scrypt buffer sized 1468006400
[2013-06-07 00:34:54] initCl() finished. Found Tahiti
[2013-06-07 00:34:54] 1 gpu miner threads started
[2013-06-07 00:34:54] Pool 0 not providing work fast enough
[2013-06-07 00:34:54] API not running - API will not be available
[2013-06-07 00:34:54] Generated benchmark work
[2013-06-07 00:34:54] Pushing work from pool 0 to hash queue
[2013-06-07 00:34:54] Generated benchmark work
[2013-06-07 00:34:54] Pushing work from pool 0 to hash queue
[2013-06-07 00:34:54] Generated benchmark work
[2013-06-07 00:34:54] Pushing work from pool 0 to hash queue
[2013-06-07 00:34:54] Popping ping in miner thread
[2013-06-07 00:34:54] Popping work from get queue to get work
[2013-06-07 00:34:54] Got work from get queue to get work for thread 0
[2013-06-07 00:34:54] Generated benchmark work
[2013-06-07 00:34:54] Pushing work from pool 0 to hash queue
[2013-06-07 00:34:54] Generated target 00000000000000000000000000000000000000000000000000000000ffff0000
[2013-06-07 00:34:55] [thread 0: 18432 hashes, 17.7 khash/sec]
[2013-06-07 00:34:55] GPU 0 found something?
[2013-06-07 00:34:55] OCL NONCE 33519 found in slot 0
[2013-06-07 00:34:55] New best share: 6
[2013-06-07 00:34:55] GPU0: invalid nonce - HW error
[2013-06-07 00:34:55] Discarded cloned or rolled work
Any help gratefully received.