TDBEdit odd behavior

I am having a behavior problem with the TDBEdit controls.  The control(s)
in question are attached to 40 -length varchar fields of an Interbase
table.  The problem is when editing values in these controls, it will stop
the user from using the full 40 characters.  Some times it stops at 20,
sometimes 23, sometimes 29, etc.  The field length for the TStringFields
involved here is the full 40 characters of the underlying table field.  If
I delete the entire value, post the update, and then enter a value into the
TDBEdit it allows me to use the full 40 characters.

Does anybody have a clue?