We have been having the same issue for quite a while and have narrowed it down to a multitude of issues.
- If you have an attachment (this one has been odd for us, it's pdfs over 200kb)
- If your sharepoint server is setup with load balancing
- If your profile (also can be called you outlook profile) has any errors in it or has gotten corrupted
- If your computer that you use infopath on has any problems related to web storage (this is a windows problem and there are kbs from Microsoft to fix them)
- If your document library has grown to a certain size, it could not be indexing correctly (which we have had happen)
- If the sharepoint server connection is through a WAN device for vpn, you can have this problem
We have noticed that when we restart our sharpoint server the problem goes away, most of our problems went away when we took off load balancing for the sharepoint server (this took months to figure out)
I would recommend getting fiddler2 or comparable program and see whats happening when you try to save. It won't make sense at first, but once you learn how to read the output, you will start to figure out the problem.