speedo confusion

i have had an intermitant fault with my speedo for some time now and have decided to get it resolved it sounds like a common fault ,whilst driving only the speedometer drops to 0  and radio volume drops and after a few seconds goes back to normal . after researching about the problem am asuming it is the speedo censor on top of gearbox needs replaceing . i took car to my local garage to confirm and they said it was the speedo drive that needs replaceing and because of age of car could cost between 4 and 800 quid dependant if they had to take gearbox out to remove drive or censor or whatever or even change v gearbox if a pin breaks v casting while being removed worst case scenario .i have an old school haynes manual and v only thing in there is how to remove and replace speedo sensor no mention of aspeedo drive any idea what they are on about i have trawled online for solution and all i can find is how to change speedo censor not speedo drive or are they the same thing . mk1 focus 2002 zetec 78k on clock