How to stop periodic rev drops on a fuel injection Punto?

I have an N reg 1.1 Fiat Punto with fuel injection, but when the car is cold, and quite often even after its warmed up, the revs suddenly drop down ever now and again. When I'm on the road, it has the same effect as if I were gently braking, then the revs kick back in and the car jolts forward again.

Sometimes, after just starting the car up, the revs will drop periodically, i.e. every 5 or 6 seconds, and will keep doing so until I floor the accelerator for a bit. Stepping on the accelerator just slightly often makes it worse to the point where the revs drop so low that the car stalls.

Anyone else experienceing this, and if so, any suggestions on how to get rid of it?