Re: Clutch Problem, Please help !!!
I had this exact problem a couple of months ago. After speaking with many Alfa owners I decided it would be the slave cylinder as lots of other 155 owners had recently changed their clutch slave cylinders too. The Slave Cyl is very easy to change and there are 2 types of these depending on vehicle year. There is one that fits with a circlip on a bracket (which I assume is the skinny body car type, which I have) and there is one that fixes with 2 bolts, which must be the later spec. To get to the item, which is mounted on the top of the gearbox you must remove the battery tray. The cylinder is then visible and easy to remove, and replace with the new one. Make sure you bleed it properly after and you'll soon be driving again. These Slave Cyls are cheap and should only cost you £15-£20.
Seeing as your symptoms are identical to what I had a couple of months ago, I think it's a very strong chance that this part is all you will have to replace.