There's definitely something wrong with that. Sounds like some contamination.

How does the ballscrew feel when you turn it by hand through the sticky section?
If the ballscrew feels notchy/gritty when turned by hand, it's likely to be something ballscrew related, but if the ballscrew just goes a bit tight, it's more likely to be something to do with the linear guides.

What are the coloured sections on the ballscrew? Grease?

Any chance of a couple photos showing the carriage assembly?