Latitude D630 - 'T' and 'M' now working

​I've got a somewhat strange one - I've got a Latitude D630 and the capital T and M keys aren't working. Lower case t and m work fine, and the shift keys are working fine with all the other letters/numbers... so we know that the letter key itself works as well as the shift key - they're just not working in combination. Any ideas on where to start hunting down a solution?​

​Thanks!​