I don't know what it is, but I have noticed a lot of european cars do this when they are old, lots of old volkswagens do it, like they wont idle until they run for a minute then it is fine, one jetta we had you would just have to hold the gas down a little for 30 seconds to a minute, then it would be fine until cold again. I don't know what causes it though, just have noticed lots of older european cars have this issue