form combo boxes freeze on the form and error messages appear

[DowningDevelopments]

ok i have a form which has been working just fine till the past few days and
now when i click on combo boxes its fine but if i go to change a value in the
box i just moved off the control freezes and not just that, a bunch of
controls that otherwise are unrelated, on the same form will be frozen too.
any ideas why this might be? someone answered the same situation with
another control a few days ago saying i should use the Nz function, which i
have in this case.

<!-------------------
On Error GoTo Error_createReference

If checkAddress(Screen.ActiveForm, Nz(Me![year], ""), Nz(Me![Building], ""),
Nz(Me![BlockNo], ""), Nz(Me![FlatNo], ""), Nz(Me![RoomNo], ""),
Nz(Me![SName], "")) Then


Surname = Me![SName]
TheYear = Right(Me![year], 2)
Room = Me![RoomNo]
Flat = Me![FlatNo]
Block = Me![BlockNo]
Building = Me![Building]

Me![StudentRef] = SetStudentRef(TheYear, Building, Block, Flat,
Room, Surname)
Me![RoomRefer] = setRoomRef(Building, Block, Flat, Room)
Me![Proceeding] = True
Else
msgbox ("You have not filled in the Surname or the Room Address for this
student, please fill it in and try again")
End If

Exit_createReference:
Exit Sub

Error_createReference:
msgbox Err.Description
Resume Exit_createReference
----------------------------------------------->

Additionally i also get the following error message on this sub:
To make changs to this field, first save the record

and the following line is highlighted
Building = Me![Building]
any ideas?