A Little Off The Top

I've got a chat application that is working pretty well, but I could use some suggestions for one aspect of it.

The incoming message field just has new content append to it as the messages come in from the server:
incoming_txt.text += [the new content]; and a new line character

After a while, the performace starts to get sluggish as the words pile up, so I'd like a mechanism to trim the text field content from the top - let's say for example I set an arbitrary limit of 200 lines - as new content comes in, old content gets trimmed. Any bright ideas for a simple way to do this? I don't want to hassle with making each message an entry in an array for instance, and I haven't been able to figure out how to set a selection based on lines and not characters.

Any insights gratefully accepted!