Hard starting on a 6v system is indeed a problem that is often reported. Using an 8v battery sort of hides the problem and is, at best, a temporary solution (which can be tough on 6v light bulbs).
As, posted above, the most common cause is incorrect or bad battery cables and/or bad grounds.
Here is list of progressively more costly "solutions" that many people have posted over the years.
Good luck.
