I'm experiencing an odd idle speed problem with my 740 wagon. After I park the car and then come back about 10-20 minutes later, it restarts fine, but then won't come up over idle speed when I go to drive off (it's an automatic). Within a minute, it will finally rev up and go, but meanwhile it just sits there or creeps along very slowly. Yesterday this happened 4 times in a row as I was running errands. After a couple of times, I just let it idle after starting and the revs dropped to almost 500 rpm and then finally picked up to 1200 and dropped to a normal idle in less than a minute. No amount of pressing the gas pedal made any difference until about a minute. The weather conditions were damp and a bit above freezing and it seems to me the previous time it did this, the weather was also wet. I can't remember this happening in warm, dry weather.

So what could be causing this and can it be fixed easily and cheaply?