My 280se 4.5 has an issue that I have no idea what might be causing it.

The car starts fine when the engine is cold. I have not tried to start it when the engine is hot. It idles fine when in park or neutral. However, when in drive as soon as I come to a stop at a red light I have to put the engine into park or neutral. If I keep it in drive after about 20-30 seconds the idling speed will reduce and then the engine cuts out.

Not sure why it would idle happily in park/neutral but not in drive. The engine does need a good tune up as it does run a little rough and does backfire mildly.

I plan on taking it to my mechanic as soon as funds become available but I wanted to know what the possible causes might be.

Thank you.