Off undersaddle, but not on lunge?

So, I was out to the barn last night to see how Gus was doing. Figured since I had canceled my lesson, I still needed to see how Gus was moving on the lunge and undersaddle. Took more videos as well.

Long story short, he is better on the lunge WITHOUT a rider and lame, essentially, with a rider. He started off okay with me up in the saddle, but as our ride progressed things got worse. He is extremely uncomfortable doing small circles more so when he goes to the right then to the left, if I'm remembering my directions correctly.

Here's the videos:

I ended up leaving his boots off, even though he seemed unsound upon leaving the arena (not sure how noticeable it is in the posted video above, video #4). Funny thing was, when I went to lead him back outside after I was done with him for the evening, he was moving sound and very freely. Go figure. I'm thinking it was because I had his evening grain in my hand. Haha.

Vet was supposed to come out today as well. I left her a note along with a link to the above mentioned videos. I'm not sure if she ended up looking at Gus or not, I left it up to her. He's definitely not comfortable so he's back on 1g of bute daily for the next week and a half, then we'll reevaluate things from there. So, here goes another long (hopefully not!) recovery.