Pickup Lunging

Anyone had this problem? When I put the gear in first, it will lunge forward backward as if its about to stall (like if you leave it in first and slow to a stop without the clutch), and at different degrees, sometimes real rough, sometimes hard to notice. Most of the time it doesnt do it at all. WTF? Is this a tranny thing? Or the engine. The engine seems to be fine from listening at idle.