Posts overlapping when clicking on tag

When I click on a tag in my site, I’m often – but not always – presented with pages whereby the related posts are all jumbled up.

Here’s an example:

http://www.tvcream.co.uk/?tag=emma-freud

In fact, in one case I’ve found, although the tag only finds one result, that’s also jumbled:

http://www.tvcream.co.uk/?tag=janice-long

And, to add to the confusion, here’s another tag page where everything’s functioning properly:

http://www.tvcream.co.uk/?tag=beryl-reid

Any suggestions?