There may be a problem with the electronic contacts under the keys - perhaps something has been spilt on the keyboard at some stage in its history, or dust has accumulated on the electronic contacts under the keys and is causing them to stick or not make good contact.
Either way, the organ will need to be stripped down and the electronic contacts under the keys will need to be cleaned with electronic contact cleaner or by wiping with a cloth soaked in alcohol solution.
Allow me to also suggest, before such disassembly and cleaning, you might try some compressed air (not spray, just air, dry air - from a can, there is often condensation in 'canned air'... so air from a compressor might be best) blown in between the keys, esp. the keys that have problems.
Sometimes it helps.