See the link above:
Plain and simple just don't use dropbox for your podcast files or RSS feed hosting if you actually want your podcast to work well with iTunes and iOS devices.
Dropbox is not meant for podcasting. There are many discussions about this on the forum boards - they all end the same way - dropping dropbox - and getting a real podcast host that can handle byte-range requests.
You are unlikely to find a free hosting service which is satisfactory. They have to make a profit somehow, usually by adding adverts to customers' web pages, and there is no advantage to them in hosting media files where they can't do that - or in providing the bandwidth for a lot of downloads.
Any competent ordinary website hosting service will do - there are many to choose from, some quite reasonably priced - and indeed your ISP may provide some web space as part of your connection plan.
However the service must provide 'byte range requests' (this is the method used by the iPhone, where only part of the file is requested at a time) or Apple will reject your podcast: you need to ask any service you are considering using whether they can handle this, and if they say no or don't know what it is go somwhere else.