Changing the value in one ComboBox also changes the value of anoth

I have a workbook with a couple of ComboBoxes, and when I change the value of
ComboBox2, the value in ComboBox1 SOMETIMES changes as well. I'll try to
coherently outline how they are connected below (here's hoping):

ComboBox2 is linked to cell IV4 of the workbook, returning column 1 of its
ListFillRAnge.

ComboBox1 is linked to cell D2 and returns the result of a VLOOKUP formula
which exists in column 2 of its ListFillRange. That VLOOKUP formula uses the
value of cell IV4 as its lookup criteria, and references one of several
tables, depending on the formula returned (which again, depends on the value
in ComboBox2).

My temporary "fix" for this has been to order the boxes so that ComboBox2 is
above ComboBox1 and have instructed users to make their selections in that
order, but this is clearly not a complete solution and when I publish this to
a larger audience I am sure to get complaints about this.

Any help will be greatly appreciated.