Air leaks would usually mean the rev's hunt as the idle actuator opens and closes to get the revs to sit at a static rpm. However it can't because there is air coming in from else where.
Bad idle you'd need to describe a little more.
Do he rev's hunt as above? (Increase decrease a lot)
Is the idle lumpy?
Do you have noticeable flat spots (Thinking slightly timed incorrectly)
If its lumpy I'd say its leaning off a little bit too much. If the mixture is rich is wouldn't cause this.
As the temp rises the fuel gets leaned off slightly. (Think old cars and the choke).
However modern systems like the Alfa in question used a MAF (Mass Air flow) and Lambda in the exhaust to measure if a mixture is too rich or too lean and changes the opening injectors to increase opening times for more or less fuel.
Checking for air leaks after the maf (Going towards the throttle body) is the first thing to check. Un metered air would cause issues.
Alfa 166 2.0 Twinnie - LPG and a few trick performance parts