Its a Peugeot 205 STDT. When l start up on a morning or after the car has been left 8 or more hours, the tachometer works as it should, but after driving for about 10 minutes it becomes erratic for a couple of seconds then falls to Zero. What is my starting point to diagnose or fix the problem?
Thanks.

Sounds like it needs a new sensor, its fitted right on top of the gearbox pretty easy to change!!

l would start by checking the speedo drive. You will find it on the rear of the gearbox about halfway up, its mounted into the right hand side of the differential housing, usually whiteish in colour & will either have a speedo cable (10mm) or 3 wires coming out of it. It will be fastened by 1 bolt which will have either a 6mm allen key head or a 10mm hex head. if it has a speedo cable coming out of it then pull out the rubber bung from the speedo drive & then pull out the speedo cable. spin the end of the cable with ur fingers & get someone to see if the speedo needle moves on the dash. if it does not then change the cable otherwise pull the speedo drive out of the gearbox. A plastic gear should come out with the speedo drive, check that its ok before u replace the drive.