cannot access mapped drives (intermittent)

K have a client with server 2008 R2 thar is havind some strange errors. On some of the workstations they are losing the mapped drives and have difficulty restoring the drives. The software they are using needs a co nection to the server to access the SQL database there. Sometimes they just lose the one drive sometimes both. Sometimes the drives come back upon a mouse click must times they don't.

Also get an error such as "name already in use". It dOes not seem to have a pattern to it. We have changed out the server and the switch. Checked the nic cards and shut off the option toputthe nick sleep when idle. Added registry keys to keep the connection open (KeepConn).

Running out of ideas. This happens on both win 7 and win 8 on a domain (erp server). What am I missing here?