You have a high resistance connection at one of the battery cable terminals. Pull both cables and CLEAN them thoroughly. Wire brush the posts and scrape the inside of the cable clamps. I've seen hundreds of identical problems, and it's very unlikely you have a bad battery.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!