Car problems

[GroomGTI]

Hi just need some advice when i leave my car to idle is sounds really bad as if the exhuast is blowing but its not. It sometimes idle revs and the odd time the car will cut out if ya leave it to idle for a while. Also when gearing down sometimes the revs will drop right to the bottom not cutting out but nearly. Any ideas what it could be.
Rep for some advice thanks :y:

[mattg]

could be lambda sensor, my mates was faulty and when he changed his it sorted it out, he had very similar faults.

[GroomGTI]

Only happen since i had an engine conversion. cheers for the answer