I found something that worked for me.
Don't asked me why it worked. It's insane.
But it did.
http://blogs.msdn.com/b/wndp/archive/2006/11/03/enable_5f00_vista_5f00_network_5f00_map.aspx
The instructions are for vista, but i applied them to my windows7 machine that was having trouble...
Finally... this is the EXACT same problem that I'm happing. I can access the windows 7 share intermittently. Usually after a reboot of the mac I'm using to access it. But ultimately, I get "connection failed" and the mac can't even see the Win7 PC.
Did you ever figure this out?