Vauxhall Vectra rpm and speedo don't work (sometimes)

so when you turn the car on either the rpm and speedo both don't work and are stuck on zero, or they both do work perfectly fine... car still runs perfect without instruments working... another thing i noticed is that if ever the instruments are working and a stall the car the instruments will stop working.... i can live without the rpm dial, but i need the speedo.... also, once the instruments don't work it isn't a case of turn the car back on they'll work, usually the car has to be off for a fair while for the instruments to start working again..,. i'm stumped as to why they'd only work sometimes.