a quick scan of the resultant pages didn't see anything helpful to the current issue (well maybe if a capacitor the size of a microwave oven is used). If the OP can't change the power supply or make any modifications to his current setup, his project is toast.
Maybe a more through scan would help too. First off, it describes the use of star grounding
that G_M originally mentioned. If properly implemented, this helps keeps the motor current
noise from getting onto the power and ground leads of the video ckt.
Secondly, the Main Capacitor needs to be placed correctly [as indicated on the link cited], and
its job is to help compensate for the inductance of overly long leads [meaning more than a
couple of inches] from the battery/power supply to the board being powered. It probably won't
help much if the motor load is so great it shorts the buss temporarily, or if it's located in
the wrong place.
Thirdly, it was pointed out back in reply #17, and probably also by others, that what OP has
been trying to do all along is find one patch/fix for a crappy design, rather than build a
better design from the getgo. Further, he seems intend on abandoning every "fix" that
doesn't solve the problem completely, whereas he's been told that a good design has many
aspects, not just one bit of magic.
Fourthly, the reason for pointing OP to the links in the first place was because some of
us don't believe in simply spoon-feeding proverbial loaves'n'fishes to neophytes. Ultimately,
the object is to learn something about what you're doing.
Fifthly, relative to item #4, it might be helpful to look up the concept of "Cargo Cult". See
especially "In an attempt to attract further deliveries of goods, followers of the cults engaged
in ritualistic practices such as building crude imitation landing strips, aircraft and radio
equipment, and mimicking the behavior that they had observed of the military personnel