2001 Tahoe Idle

Hi, this is my first post on this forum. My 2001 Tahoe has started to idle down when I am stopped. It kind of goes up and down, not to the point that it is going to shut off, but just enough to be anoying. It doesn't do it everytime, but it happens fairly often.Does anyone have any suggestions on what to do?

could be the alt. or the torque conveter, but i highly doutb its the converter, but it could be