Speedo doing strange things Drifter B2500

Hello. I have a Drifter B2500 Double Cab, which I got about 4 days ago. I noticed while driving, the speedometer would drop to zero for a second or two, then jump back up to the speed I am doing. It does this intermittently and not very often. I assume it's a loose connection somewhere, or a faulty sensor? I also assume it's an electronic connection, not a mechanical speedo cable? Am I correct? Where does the speedo sensor live? I read something about it being on the rear diff, or on the gearbox? I hoped there was something easy I could check before getting stuck into dismantling the dash etc. I wouldn't even know how to get to the speedo, the dash looks like it's one complete moulded unit, not too many screws. Help would be appreciated.