error codes Engine 17978 , Immoboiliser 01176

[f1basgh]

I have a Year 2000 Polo 1600cc Petrol Estate S ,and when I put my VAG Scanner Code Reader it keeps coming up in the engine section with error code 17978 - P1570 Engine start blocked by immobiliser . Then in the Immobiliser section Error Code 01176 Key - Sporadic Signal to low . Only thing is the car starts every time 99% of the time , just every so often it starts then stops immediately but will restart the next turn of the key . Any one any ideas please . Tony