might be a silly question - but are you sure the 'new' keyboard works ok elsewhere?
Does the 'old' keyboard work elsewhere?
Do the num lock/caps lock/scroll lock leds light?
If the fault is definately not the keyboard, would suspect the problem is on the motherboard.
On the mobo, check the connections on the keyboard socket, maybe a dry joint has developed.
As a matter of interest, where are you located? I am in Hammersmith, West London.
Good Luck - keep us posted.