Red X on mapped drives after few minutes

Win 2000 prof client logs onto NT 4
PDC nicely with logon script.
“My computer” on the client then shows
the correct mapping letters.
After about 5 minutes of inactivity
(only using local C: drive),
“My computer” now shows RED X.
Clicking on one reveals error – drive cannot be found etc. – only after repeated clicking does mapped drive
awaken and open up. – I did see that
right clicking on connection has some
“idle and sleep properties” which I
of course tried all possible combinations of checking and unchecking
Can anybody explain how do fix this