Hello Dom,
Before you check out the keyboard, inspect the Search Unit PCB for any sign of cracks in the tracks, if there are any more than likely they would be on the side where the long connector from the keyboard plugs into it.
If your wiring is the older type that plugs into the top of the search unit rather than on the side (later jukeboxes) to make sure that all the connections are good, meter back to the keyboard if nessary.
If all looks and tests out OK try cleaning the keyboard switches on the effected letters/numbers, my guess it is the letters that are the trouble. You can use alchol in the pubs I used to use Gin and a small brush or you could use tape head cleaner. Do not use any form of switch cleaner, it will not work and may leave a residue.
Please us know how you get on.
Regards
Alan
Alan Hood
ami-man
UK