BERLINGO REV COUNTER ACTING ODD. PLEASE HELP!

[kb]

Please can someone help? My 2001 Berlingo Multispace Forte 1.9d, its rev counter has started to do strange things. Driving along one day, I noticed my rev counter started to work backwards, the needle was registering below the numbers on the dial. It then seemed to reset to zero, but only move occasionally upto about 10 on a gear change, then immediatly drop again to zero. When driving along it also appears as though it wants to move from the zero flickering constantly, its like it doesn't have th energy to move. I don't fancy paying Citroen £30 just to plug it into their computer to find out what is wrong, without paying for the repair. If it is something simple please advise with child like directions as I am not too familiar with an engine bay, I know where the oil and water go and not much else. My hubby is already complaining about my choice of car, with lots of niggly things going wrong, don't want to admit something else has broken! So please, please help.