I'm not so sure about that "cannot be change" part. Somewhere in the code the brower is told where to store temp files, so at least in the case of mozilla this behaviour could be changed to another location (of course you'd have to recompile it).

The part of his question which is most bewildering to me is that this only started after he switched to broadband. So could one infer that this is not the behaviour with dialup internet settings? And if so, why?