300tdi Speedo Problem

Have a 1995 300TDI auto. Speedo reads find most of the time but occasionally goes very erratic. Revs stay constant but speedo flicks up then down to the right speed. Erratic then fine then erratic again. Have checked connections both ends of wire all appears fitted correctly, no loose bits. Before I start buying sensors and speedo has anyone got any ideas please. Doent usually happen till engine warms up!!!!

For anyone who has a similar problem, I did eventually trace the problem to the sensor pickup on the side of the transfer box. The two parts of the plastic sensor housing had seperated slightly allowing an ingress of water when driving through puddles. Hence the erratic nature of the problem.