I think the gel looks perfectly normal, your samples are a bit overloaded. Try reducing the amount of DNA per lane about 3 fold and see if it looks better.

I'll try that, thanks

Overloaded gel. Is the plasmid cut? Sometimes excessive salt in the plasmid prep (if pptn is involved) can cause some smearing / tailing.

I don't believe so, the plasmid is a straight from the manufactorer puc18 plasmid

Try as Mike suggests - you could probably get away with 1/5 the amt you are currently using - no point in wasting expensive bought in plasmid.

I've been told by labmates that uncut plasmid DNA should be run on a gel without ethidium bromide because it messes with the migration of circular and supercoiled forms. This seems to be the case in my hands. You can then soak the gel after running in a little TAE with ethidium bromide (empty tip box works fine).

Were there any differences in the running buffer? If it's a new batch then you'll want to make sure to use the same batch for making and running the gel. You could also try a lower voltage to obtain straighter bands.