I have been around but not used sliding STALL doors, though my barn has a sliding door to the adjacent shelter. The sliding doors are really space economical and probably, in the long run safer with most horses. I have trained my horses to put their heads over the gates to get haltered/unhaltered, and two of my stalls are constructed from round pen panels, 3 sides + security gates 

I had gate issues for about 3 years with mine from ulcers we couldn't get under control. Started just stopping and refuisng, sttod like a statue. After a few minutes he's finally just go. Escalated to him pinning himself against the alley panels, wall, whatever there was. Then he started to try to climb the panels or walls. One place he pinned me into a truck about 100 feet behind the arena gate. Ended up with his front feet on the hood trying to climb it. He'd do all this & then just all the sudden rocket off and go. After trying what seemend like 1000 differnt things I found the magic combo that worked and kept the ulcers away. It was my last attempt, if it didn't work he was going to the pasture. The gate issues didn't immediately resolve. I gave him about 5 months off, did the treatment during that time with the daily prevention. Started the season and the gate issues were there still. After a couple weeks I had him scoped again and he was clear. So we thought, ok it's ingrained into his brain that it hurts. Slowly it got better every run until finally after maybe 2-3 months they stopped. Hasn't had a problem since, we've made 2 full seasons now going in fine. So if it is a pain issue for yours, just make sure you give his brain time to adjust and realize hey, this doesn't hurt anymore, I'm ok. If it is a fragility issue, then that's another story but also is just time. At that point it's building confidence. With mine sometimes I go to local fun shows and sign up, $3 a class, and just go in and trot a pole pattern or flag class, or just whatever isn't barrels. They kinda think, ohh well I don't have to run all the time when I go through these gates. I think if you have access to that it's good. Even for any horse. At 5 years old, he shouldn't be too far gone to fix, whatever the issue turns out to be 

Mine just about blew up years ago. Had her fully vetted, found, fixed and still maintain the issue. She's MUCH better now. That said, when she gets in certain situations as far as alley set ups, if things feel claustrophobic to her or other horses are getting out of control, she will react to it. Not near has bad as it was by any means. She will balk at the alley if it is tight and back a few steps, but as soon as I disengage her hip and nudge her forward, she will go. 

If I weren't properly maintaining her I'd  think pain. But it's been years of proper care. I really just think mentally, certain set ups are jarring to her. I try to ignore it as much as I can and not make a big deal of it. 

I agree that if this is a new issue there is something behind it. Whether horse or rider is something to be figured out. 

I want to opine that we barrel racers really don't generally want our horses walking in flat footed -- we all understand that the fast twitch muscles need to be engaged at least in the alley, and most horses, over time, know that they need to get those muscles going.  Thus the saying 'on the muscle.' I've not seen many, if any, 1 or 2D horses walk in flat footed for years and years. 

That being said, I've watched a rider start to have issues because their horse started to get on the muscle and call it an issue and go on to make it a true alley issue with wrong cueing when the horse got on the muscle. If you yank or pull back just because the horse gets on the muscle it will go backward, sometimes spin, and sometimes pop up. Holding a horse is not the same as pulling or yanking. 

But if a horse goes from solid forward motion, whether on the muscle or not, to turning away or popping up out of no where, then there is either a rider queing bad or a horse in pain. 

Sometimes the particular show set up will make one somewhat more apt to have an issue. Zan is fine except when we are in the group of 5 separated from other horses ready to go in. I can feel his heart go and he gets way sensitive to any movement of mine. Some is his feeling my nerves. But he will turn sometimes. I have worked hard on my breathing and that has helped more than any pressure added to Zan. If he turns I kiss and let him keep the motion up until we are turned back and then kiss again - giving him rein slack at the same time. Less pressure, not more.