Engine stalls after 30+ minutes of driving when I put it into Drive

Something that has bugged me since I have owned my car is after the car is warmed up, and I have been driving it for 30+ minutes, if I stop, put it in park or reverse, it stalls everytime I put it into Drive.

Any ideas why this is happening? Wasn't sure if this was a vacuum problem or tranny problem or both.
[addsig]

What stall convertor and check for vacuum problems on any lines running to the tranny. I know there are other things to check but these come to mind right off. Might be able to work around with a higher idle speed.