Air Conditioner Problems

I just purchased this van. 1998 Ford E350 Club Wagon Triton 5.4l. With the temp control set to cool and function control set to either normal or max cool and fan on high, air will blow through the dash vents durning idle or a little increase in speed, but with more increased speed the air stops blowing out of the vents. I can still hear the blower continue to work, but at that point it seems to blow out heat. Also the rear air doesn't seem to work either at that point. Thank you, Chris LaRue 2006 E-350 with similar issue. Without me touching the conrols, air blower will divert from dash to floor to defrost to somewhere else. Also seems like hot air from the engine compartment seeps in when its cycling around like that. This is somewhat sporadic, but with the heat in Arizona, its hard enought to keep the van cool when its working correctly. If anyone has ideas or suggestions, please let me know. Thanks, HW