Random electrical issues

[Still Thinking]

Strange random electrical issues have started to happen with my 2005 Mercedes C320 saloon. The clock will not keep time, on starting, the dashboard lights sometimes do not come on but the headlamps and rear lights do even though the light switch is off. Random error messages appear such as ‘SRS take car to workshop’. Sometimes the issues right themselves after a few seconds and sometimes I have to switch the engine off and restart it to clear them. It is entirely random and it may run for several days without issues. I’ve noticed that it happens more frequently when the car has been left overnight and is restarted the following morning. The battery, amazingly, is the original factory one fitted so nearly 14 years old. I had it tested recently at Halfords and they said it was a ‘good battery’. The car always starts on first turn of the key. A new alternator was fitted about 4 years ago. Any ideas? Thanks.