Those symptoms are generally an indicator of the alternator not charging. In addition to checking your grounds, check your battery cables for corrosion and good connections at both the battery and alternator. You might also check your alternator belt for condition and proper tension, an intermittently slipping belt could cause your symptoms. The problem could also be in the alternator itself - an intermittent short or some other weird internal problem. That would be hard to diagnose unless you can catch it while it's happening.

95 Montero SR
3.8 MIVEC, Advance headers, 2 1/2" exhaust, Magnaflow muffler, OME shocks & rear springs, 2" body lift, 3" tank lift, 4.90s, TRE front locker, factory rear locker, Roger Brown Rock Sliderz, 315/75r16 (35") tires, Sport big brakes