Contamination must be the cause (?). Anything else they should be able to detect, such as fails in the wiring. It's hard to see what it could be that they couldn't find by monitoring through the diagnostics port - presuming they know how to read the output (they are Alfa specialist aren't they?). If they are reading the codes and see 'lambda failure' that may be because the wiring has a fault, not the sensor itself! The sensor wiring should be checked, especially the connectors tot he car. This includes the heater wires.
Are they genuine Alfa lambdas - if not try one.
There are a number of causes of contamination, be they metal oxides, silicon, oil or petrol.
If you are running too rich then the excessive unburnt fuel will kill the sensor quite quickly. Try an injector cleaner additive for a few tanks of petrol. Carbon fouling can arise from too much fuel (black end to the lambda). With lost of short journeys you may have what is known as 'Housewives Car'
. That needs a good 'Italian tune-up' to clear it off
(a darn good run down the motorway and back). It could also be that the MAF is going and the lambda is compensating and overfueling the car. Too much fuel can also wash the bores and remove oil into the exhaust gases plus wear your engine out faster. Check the cars emissions as this will give you needed clues.
How is your oil consumption? Alfa's do tend to use a bit but is your excessive? An shiny black tip on the lambda is pretty obvious.
Never spray silicone based products into the car throttle (WD40 has silicone in it apparently). Only ever use Carb cleaner. If you have silicone contamination you will have some white or light grey coating on the tip of the sensor. If you have had a repair recently where silicone sealant was used that wasn't lambda safe then this could be your constant contamination. Typical areas would be sump repairs or anywhere where the seal was in contact with the oil.
Can you describe the symptoms that make you/them think the sensor is failing?