What causes the momentary dip in headlights when you put the car in gear & drive off after starting the engine?

If your talking about the slight dimming when you put it in gear and then brightening when you accelerate, it's battery voltage that hasn't steadied out after starting. Look at your voltmeter next time ans you'll see a dip in the volts and then a gain when you hit the gas.

This is normal.

Larry