speedo on 53 zafira erratic and power loss?

when i start the car up in the mornings and at night after work the speedo does not work and the engine cuts out if I take my foot of off the throttle. This happens for about the first 10-15 minutes of driving then is OK. The speedo is still on and off though. Speedo is not a problem but cutting out is, especially if it happens on the motorway at 70mph.