Not a lot, a head gasket going on a car is usually the be all and end all of it. The head gasket is like the heart of the car and i don't think constantly replacing it will continue to work, well it hasn't in fact. Don't know why it won't work, but if it's not cracking and warping then there is obviously something else amiss - like the plugs but it's like giving someone a kidney transplant when their heart is worn out - pretty pointless. You can't really blame the garage for washing their hands of it, replacing the head gasket is a massive task as you have to practically take the engine to bits, if they don't knwo what's wrong with it, they're probably wasting time/effort/parts replacing something that is inevitably going to to break again, or replacing something else that isn't going to make a difference. Out of interest how old is the car? have you taken it up with Fiat?