plasmid multimers?

[Hanbok Kim]

After I moved to a different research place, one 
phenomenon bothers me. Intact plasmids before 
cutting with restriction enzymes are located in
a 20 kb position, although its real size is 2.9 kb.
It is pUC18. After cutting, the plasmid is seen at 
the right position. However a complete digestion is
hard, and some mutimers (?) still remain in upper
position. How can I prevent plasmid multimer?
I would appreciate your advices.

Han,
hbkim at biochem.kaist.ac.kr