Thanks Jan appreciated the fast response, and realizing that reporting the issue without any extra info isn't make it very useful for you guys.
So will make the next time a more detailed report with the relevant screenshots...
Ps above was with Airac1606 no hardware jitter problems here, it was after selecting a new arrival and runway and after the next way point lnav disconnected,
Which FMC software version is simulated so I can compare with CBT or the FCOM....
Or is that mentioned in the manual lol will have a look...
Cheers,
André