2013 Transmission "hunting"

Looking at a 2013 Explorer xlt 4x4 a client of mine has. 130k mi and it just started to hunt for gears. Driving along in any gear with pretty constant pedal on flat ground and the tranny is hunting up and down. I don't think it's the shuddering symptom I see so many complaints about online. just feels like an intermittent gear shift, sometimes making a perceived driveline KLUNK . I know its a long shot but would anyone have any thoughts on what could be the issue? I Drove it for a half hour, no codes at all. Will a tranny flush and fluid with friction modifier help?.. he is looking for $8k for it which seems like a super price, vehicle is one owner very clean.. I love the truck, it drives great, is responsive, comfortable... Feels much better than my hulky 2007 Expedition EL but if it needs a new tranny I'm probably not interested. Someone please give me some ideas ...thank you in advance!