Engine surges when in drive but stopped

What is your question?

When I'm at a stop sign or red light and the car is stopped but in drive the engine will surge up and down kind of like an ebb and flow of power. When in park and idling - I don't notice this, nor does it feel like its doing it while driving. Any ideas?

What makes this problem better or worse?

Surges when in drive but at a standstill

How long has your 2004 Mercury Grand Marquis had this problem?

It just started yesterday