Lurching at idle

When the vehicle is running and in park it is fine at about 900 rpm. When shifted into a forward gear it drops down to about 500 rpm and feels like it lurches from side to side. Once going at speed limits it seems to run fine. It does it when you stop. I had gassed it on the highway and it all stopped. Spark plugs and the wires were just changed recently. Any idea if something is wrong? Planing to drain and fill transmission fluid. Have not checked the harmonic balancer or engine mounts. The lurching is not violent but also not light.