Do the voltage test during cranking, and I'll bet you either have an under-capacity battery for the demand, or a bad battery. Just because the battery is new, or in good condition, doesn't necessarily mean it has the current delivery capacity to handle a starting load. If you're below 4.5 volts or so under starting load, you need more 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!