2002 Mazda Protege Charging light

I've been having a problem with my 2002 Mazda Protege. The check battery light is coming on intermittently. I get home at night and the light is on. The next morning I start the car and there is no light, then I drive for 5-10 minutes and the light comes on. (Due cold engine?) If I stop the car and start it again sometimes the light comes right back on and other times I can drive for 1/2 a hour before the light comes on. I've checked the output of the battery several times when the charging light comes on. Sometimes it's right at the pass/fail line of the meter other times it reads OK but the light is still on. I've had the alternator tested (it's OK), tightened the belt and cleaned the connections any ideas?