Engine running bad in stationary

1998 Ford Explorer Black Edition, 4L V6, 180.000 km, automatic, runs on both gasoline and LPG.
When starting with a cold engine, the car goes high in rpm and falls back immediately and is not able to keep it straight (the engine goes up and down between 400 and 1000 rpm). I have to keep my foot on the gas to keep it running. The first few miles also, for example when waiting at the trafficlights, I have to keep one foot on the brakes, and the other on the gas. When the engines is warm, there seems to be no problem, and it will start properly and run quite normal in stationary. Although it runs to low, about 500 rpm. The car seems to have trouble with the low rpm and starts correcting, so it goes a little bit up and down again in stationary.
We have tried everything you could possibly think of, but nothing seems to help. The problem here in the Netherlands is also that no one understands American engines. They are a lot different from the European and Asian ones... I really hope some one recognizes this problem and knows how to fix it, because it is such a great car and I really don't want to get rid of it. The problem is the same on both gasoline and LPG by the way.