over 2000 comments on one post causing date display errors

I have a couple of very popular posts that have attracted over 2000 comments – and the comments have started to display in the wrong date order, so comments from a couple of months ago will be jumbled in with the most recent ones. I currently have display set to 50 per page, last page default, older comments at top.

Last time they got to this level, I just deleted the oldest 1000 using wp-admin, but it didn’t solve the problem at all, and the comments were still in the wrong order. Eventually, about a month or so later, they seemed to fix themselves somehow (?!). Now they’re getting up to the same number again, I’m thinking I’ll need to do another ‘clean up’ – how can I do this and preserve the correct order?

Do I need to “remove overhead” from the database afterwards?