#REF appearing when cells,rows,cols are all protected
 

I am working on a excel staff structure that has an issue with
changing formulas. All formula cells are protected and half are
hidden
but every so often a reference will disapear from a formula. It is
never the same column or row and is only used by only 1 person per
shop no other interferance can be traced. Sometimes it can occour on
a
page that has not been modified.
here is a copy of the formula
=Hours_Worked($A81,C81,C82,$A$2)
hours_worked is the vba part of the program to work out the number of
hours worked and the remainder are reference cells
=Shift_Pay($A81,C$4,C81,C82,$G$2,$D$3,$A$2) +$K84/7
shift_pay works out the pay rates and any penalities in vba and gives
a $ figure

=Hours_Worked($A81,#REF!,#REF!,$A$2)
this is what I get which gives the errors
=Shift_Pay($A81,D$4,#REF!,#REF!,$G$2,$D$3,$A$2) +$K84/7


it is very random when and where it happens, It's like there is a
ghost in it doing what it wants
can any one help me?