I reckon this has got to be to do with how the range updates itself when you insert a row
It may be a better idea to iterate thru the list and use the additem method ie
Combobox1.clear
for each c in range("NamedRange"

combobox1.additem(c.text)
next
This really doesn't take very long
The other way round is possibly to use a "Dynamic" range name ie goto insert > Names
Put a name in and then where you would normally put a range ref, enter:
=OFFSET(Sheetname!$A$1,0,0,COUNTA(Sheetname!$A:$A)-1,1)
Where your list is in column A, starting in A1
This will update as you enter data at the bottom of the list and you will therefore not have to insert a row
HTH Rgds
~Geoff~