I had a similar problem with my 2.5 -also got bad after a "good wash". Turned out to be leaking UIM gaskets. They're figure8 shaped o-rings between upper and lower intake manifold that eventually deteriorate with fuel, oil, and heat changes. Check by spraying WD40 at mating surfaces of upper and lower intake manifold -the WD40 temporarily "seals" the gaskets and ur low-speed will smooth out. Not a difficult job to replace, & gives U a chance to clean all the EGR and intake passages as well!
Good Luck.
Ed
