Studio 1555 keys don't work (but keyboard is OK)

I have a Studio 1555 with a keyboard problem that’s seems like others already reported, but this has a very confusing twist. The built-in KB has some keys that don’t work – in this case the cluster including the enter, Pg Dn, down/left/right arrows and the bottom row of characters. I’ve tried a number of fixes taken from these forums, including:

• Using the laptop with the onscreen and USB KBs

• Draining the flea power

• Re-seating the KB ribbon connector (many times)

• Checking BIOS version (version A13) and the keyboard driver (version 6.1.7601.17514). I even re-flashed the BIOS to this latest version in case of corruption.

• Removing the battery and Power adapter then pressing holding the power button for 30-45 seconds (with battery and power still removed) before booting in safe mode, powered using in turn the battery alone then power supply alone.

I’ve also run the ePSA diagnostics, which reports Error code 2000-0411 (Cables – Keyboard not detected). At this stage it would be reasonable to conclude the keyboard (or connector ribbon) is fault, but here’s the rub. I have two identical Studio 1555s, and when I switch their keyboards over I find that both KBs give the error in the bad laptop but BOTH work fine in the good laptop. So the KBs are both ‘known good’.

I’ve inspected the connector on the motherboard closely and it appears to be OK, and knowing of no further diagnostics I can run I’m stumped. Can you please help ?

Thanks in advance.

Spec is:

Studio 1555 with 512 MB ATI Mobility RADEON HD 4570 integrated on system board

Windows 7 home premium

T9900 core duo

4GB RAM

120GB Corsair Geforce GT SSD