I have had the same problem about every 30-50,000 miles. Found this video that explains the symptoms and fixes very well. ; http://www.youtube.com/watch?v=Ev1-kEZL3qQ
Hope it helps!
It appears to be a very common problem mine was a plugged up differential tube some carbon would get sucked up into the tube and cause the P0401 cleaned it with carb cleaner seems to be working much better still using the same DPFE valve. I am driving to Phoenix int eh morning from Lancaster, CA. It should let me know if it fixes the problem.
If not Autozone has a new valve for$30.00 if I have to I'll pick one up on the road.