Have you tried dialyzing the "purified" protein after elution into a no imidazole buffer while doing TEV cleavage, and then reloading on to a His column to remove the tag and TEV? I've found in my experience that this often recaptures a lot of the background proteins.
If you're getting a 70kDa contaminant, may be a chaperone. Try washing during your purification with a buffer supplemented with 5-10mM ATP + 5-10mM MgCl2, incubate for about 5-10 min on ice the resin with the buffer, and then wash it off. Last option is to do ion exchange. On a really fine gradient, you may be able to separate your contaminants from each other.. Best of luck
