You did all that is possible, except:
1. Check that there is no interference between transmitter and keyboard.
2. Ensure that the transmitter is not near the CPU.
3. Go to device manager, delete the keyboard and reboot for Windows to detect it again as new device, and reinstall the driver.
If it still does'nt work, it's not worth your while to send it for repair. Logitech is assembled in China and spareparts are not carried anywhere. Actually, a new keyboard is cheaper than getting it repaired.