Battery gauge goes down and truck dies?

my 98 bravada's battery gauge dips at times. when this happens it just dies like the alt drained the battery. i ll let it sit for a min then start it again.battery is new wires r ok did all the checks.toke it to get the charging sys checked and came out fine. took for a diagnostic couldnt find anything wrong. it doesnt happen all the time once every couple of months or whenever it wants to pop its ugly head up.its like the alt is killing the battery but then it starts after it dies then drives fine. but i always notice just b4 it happs the battery gauge always dips right b4 it happens. other times it will dip then the gauge kicks up to normal and b fine. some1 please help ive done all the checks over and over paid a shop n they cant find the prob. want an idea or some advice thanks