No, it's not a tachometer problem, it's a secret message from the CIA agent code-named TACHO for you to meet him code (meaning the usual number) of miles/km (depending on your current place of assignment) from your previous rendezvous point. Probably they suspect that the previous agent, JACKO, will be out of circulation soon.
TRANSLATION: I haven't got the slightest idea what those messages mean either but let me take a WAG at it.
The On Board Computer (OBC) uses various sensors to compute all of that good stuff (and other semi-useless data). I'm thinking one of the car's sensors might be bad. Study the Bentley's OBC section and look to see what sensor information comes in and maybe there'll be a clue there.
Since you seem to have electrical problems you might try the "reset fix", which is to disconnect the battery for a few minutes. Make sure you know the radio code before starting.
You may find more help at
www.roadfly.org as well. Good luck.