I would say you are looking at a can fault on the bus network or an internal fault on the instrument cluster.When the fault occurs,give the top of the dash a good bang,just above the speedo pod.If this resumes the system,then I would think there is an internal fault in the pod.These are commonly fixable.May also be a good idea to check the large multi pin connector (lear type)with fold-over lock down bar,located behind the glove box,clipped to the wheel arch,behind the sound insulation.Can wiring to the dash runs through this.