1997 Fiat Punto Punto engine dies

My daughter has a Fiat Punto Convertible (R) Reg, which runs perfectly normal 99% of the time, but on occasions the engine will lose revs and dies for a few seconds (this can be at 30 miles an hour or at 60 miles an hour or even at traffic lights there is no specific trend). When the engine is restarted the it will run fine again.

We are thinking it maybe something to do with the immobliser? if so can this be by passed?

Thanks

Belinda McDonald