2 specific keyboard keys won't advoance the cursor when pressed on

Here's an odd problem. The tilde and quotation mark keys are behaving oddly.
When pressed once nothing happens on the display. i.e nothiing is dispayed
and the cursor does not move forwards one space.When pressed twice the two
characters equating to the key pressed display. if I press one of the faulty
keys followed by another key then the characters for both keys are displayed
and in the correct order.
The keyboard I was using up until yesterday was a Logitech LX500 cordless
set ( with mouse)
I contacted Logitech as I thought there was a keyboard problem. They walked
me through a troubleshooting routine but that didn't help. They then assumed
the keyboard was faulty. I exchanged the keyboard/mouse set for a new
Logitech MX 3000 set but found exactly the same problem.
Today I borrowed a MS corded keyboard and found exactly the same problem
with the same keys.
Clearly it's not a keyboard problem
Anyone have any ideas where I should start looking for a resolution.
thanks
Ted