Check and clean all your battery connections, especially the small ground wire going from the battery to the body. Watch the volt guage while its running (if you have one) to see if is low. It should read higher running and charging than not running. If it reads lower, then that is likely your problem.

'97 4Runner, '06 F350, '86 4Runner, '05 WR450
http://home.4x4wire.com/erik