DFS share disappear

I have 2003 DC holds the FSMO and 2008 R2 as additional DC, one fileserver holds the DFS root. Recently users reported that the DFS network share disappeared intermittently, I google it and no hints I could find, so can someone here please help. All our users use windows xp wtih SP3. I tried diagnostic on their PC, I found that when I ping the domain name, there are not able to resolve it. But not all PCs got the same symptom.