Quattrolvr

I have an '05 4.2L Allroad with an auto/tiptronic trans. Lately the car will not move from a dead stop if the selector is in the D position. If I flip it over to the tiptronic mode, works fine. I can even put it back into the D position after I shift into 2nd and it drives and shifts fine until I come to a dead stop. Then I have to use the tip mode to get going.

Any ideas for me? I suspect a soleniod or pressure valve but don't know how to check those.