Why does my oil pressure drop and vehicle feel like it's going to die when at a red light?

At the red light, the pressure will drop n fluctuate, then the other gauges drop n it feels, like it wants to die. Idle has a slight roughness, and the oil is at a good level. It's almost as if the alternator is bad and doesn't wnts to charge without help from a higher idle. But I've never seen that affect oil pressure. Please help!