if cleaning the drum hasn't helped, perhaps you might try a brand new Dell toner, purchased from Dell.  perhaps the toner cart has become faulty.

 

incidentally, are there any smudges on the back of the paper ?  if so, the transfer roller and or fuser may need to be replaced.....

 

?

Thanks. I did exactly that, even though the toner that was currently in the printer wasn't old I changed it out with a new one and no more smudges.