Missing last character using internal keyer

[Paulmcl]

My setup is using the internal keyer on the RRC control unit.  Works fine sending CW most of the time but will occasionally start dropping the last character on the CW message. Once it starts dropping the last character it continues until restarting the RRC control unit which fixes the problem.  Occurs with both Log4OM and N1MM.  You can also hear the missing character in locally monitored CW in the headphones.

Any ideas please?

Thanks in advance

Paul