Cylinder dropping out

[Diesel]

Hello everyone, new member here. I don't currently have a motorcycle but have been looking at a couple xj600's. One I am looking at is a 1995 with almost 70k miles on it. Seller says valves and seats were just redone along with rebuilt carbs. Problem is once it gets hot it drops one cylinder, does anyone have any info on this, possible easy fix or stay away from and keep looking he is asking$1000 for it.