I'm not using jPilot myself, so I wouldn't really know how to deal with the things you've asked
As for the garbled code, I really don't know why it happens. I found some stuff about turning of HTTP 1.1 stuff in browsers (in IE it's one of the many things under Tools/ Internet Options/Advanced). That would disable the usage of gzip between the server and clients. But older browsers don't support zipped pages at all, and if those have the same problems, then using http1 instead of 1.1 isn't going to help.
Simply put, I've no idea, and since it only affects a small number of people, I'm thinking it's some server thing instead of clientside.