seed dna gives horrible southerns

[koebner at BBSRC.AC.UK]

I have been severely disappointed with Southerns obtained from wheat dna derived
from seeds by ctab extraction.  Although the dna looks undegraded before restriction, 
and gives a 
single hmw band when unrestricted dna is hybridised, restricted dna gives profiles
which have a heavy background all the way down.  It seems as if something in the 
dna prep is messing up restriction.  I have tried cleaning up by further etoh 
pptations and also phenol chloroform extractions, without improvement.  What can the 
contaminant be?  And how to get rid of it? At least it doesn't interfere with pcr!