Vauxhall Astra speedo intermittent and irregular idling revs

For a little while, the speedometer has not worked intermittently. Whilst driving, it will suddenly register a speed of 0mph, then all of a sudden, go back to the correct speed.

I think the next problem is related... the car now occasionally has an irregular idling speed - sometimes when slowing down for traffic lights, the revs drop away and the car stalls when it shouldn't and sometimes when starting the car, it will not start without using the accelerator to over-rev. When idling, the revs sometimes waver from between 900rpm to 1500rpm.