I have a Windows 7 host workstation, and I'm seeing the same problem as the OP here with both Windows 8.1 Pro Preview and Enterprise Preview guests. Networking works great with all other guests, including XP, Vista, Windows 7, Windows 8.
It's as though after a while a resource gets used up or packets start being hopelessly corrupted. Likely the protocol is retrying, and somehow getting things through once in a blue moon. Transfer speeds can drop to as little as a few bytes per second.
I have Bridged networking, and know a thing or two about setting up Windows and networking. It's not a basic configuration issue.
I suspect what is needed is a VMware Tools update. Perhaps the networking driver just isn't compatible with Windows 8.1.
Reading the above thread, it's very depressing. Basically the help received on the VMware forum was "don't bother using VMware". Look, if you're tempted, please don't suggest "use another virtualization solution". I need VMware to work. Come on, folks, the Windows 8.1 previews are no longer new news. There must be a solution or workaround to this. Maybe a beta VMware Tools version or something in development?
Help please.
-Noel