Mapped Network Problem

hi

i am shiblu rahman , m a network administrato of a company . we have a domain and 3 servers in our company . around 100 pcs are connected to a same DOMAIN . now a days i am getting a problem that sme of the PCs are disconnecting mapped network drives many times automatically . if i logof & Loggin it is working fine , always the same problem is showing in some computers , if anybdy can help me please ………..