2010 La Marzocco Linea AV chronos will change every 20 shots or so ?

[jasonak]

Hi
Have a 2010 La Marzocco Linea AV the shot time is set to 22 sec every 20th or so shot pulled the timer will go to 15 seconds and stop or sometimes jump up to 45 seconds and stop. Then we have to reprogram it and its good for another 20 or so shots. This is a 3 group machine and its only the one head doing it. The others work fine.
Any ideas on what would cause this?
thank you