I don't recall doing more than 200 imports at the most with Lightroom, but so far I have never had any problems with import.

You may want to take a look at Downloader Pro, it is a really neat utility, that can help you organize your imports.

If you like to put images in folders named with the date and event, like:

2007-05-01 Ann and Peter Wedding

Downloader Pro will prompt you for the label ('Ann and Peter Wedding' and then append that to the date, create the folder and import the files into that folder. You never have to browse to the right directory, create the folder manually etc.

After the download, all you have to do is Import into Lightroom from the current position of the files.

Could it be your card reader that is not stable ?

Regards
Michael