This is a communications problem between the DBV and the slot.
The most typical cause of this problem are cracked solder
joints on the DBV connector where it solders to the DBV
circuit board.
Try this: take the DBV head out of the slot. then there's
3 screws that remove the bottom metal case plate from the
head. This will expose the lower circuit board. You will need
to bend back 2 metal tabs and remove a transistor heat
sink screw to get the board out of the metal casing. Using a
fine soldering iron, resolder the male communication
connector pins at this board. These solder joints often crack
from plugging/unplugging the communications cable, and
just from general vibration of the head when it "turns".
The communications plug is the largest plug on the DBV.
As facing the game, it's on the left and behind the (2 wire)
power plug. It's usually a black plug with 6 or 7 red wires
(opposed to the white power plug that has just 2 wires).
Most problems with DBV heads i have fixed by resoldering
these connectors pins and by updating the firmware.
If you need the latest Wms/Bally firmware for a DBV-200 head,
just let me know. It's version 2.70, and accepts the colored
money (including $5 bills). You will need to burn a 27040
EPROM to make that work.
The picture below, though not really addressing this exact
problem, shows the large serial connector that needs to
be resolder.