Are you printing just one of these at a time? Assuming that you are as well as that (due to your description) the long piece is being printed upright then the issue is likely that the print is just not being allowed to cool enough before the next layer is layed down. I have seen these problems before on some of my own prints. The part isn’t getting proper cooling throughout the print, but the problem is exacerbated at the top.

The easiest recommendation is to print 2 or maybe even 3 at a time, even if you only need 1. By printing a couple of them, you will give each layer sufficient time to cool a bit before laying down another layer of molten plastic.

Give that a try and see if it helps.