XP 64bit drops mapped drives

XP Pro 64 bit on domain...

When building a new comptuer for a user...and logged in as that user...we normally map a drive as "different user"... which allows us techs access to the drive we need...

Works fine, until we reboot...

When we try to open the mapped drive, we get an error...

Z:\ is not accessible
Access Denied

We have to "disconnect" then remap the drive...

Any other mapped drives., whether by hard mapping or by script work fine...these are mapped drives for the user...

We do not have this issue with XP 32 bit...

Anyone know a fix for this...???