There can be so many things causing this. Ulcers can cause the spookiness. Was for my daughter's horse, she had hind gut ulcers. Once they were taken care of no issue anymore. My gelding it was his feed in that he was allergic to corn. His body was kept on high alert as he tried to fight the allergy. I had tried ear plugs for my gelding when it first was given corn before I knew of the allergy. In his case he would either just shake his head the whole time trying to get them out or would spook when he saw another horse doing it. I then thought it was me, and also tried ear plugs on me. But, that didn't work either. Thought maybe pain when he got better after leaving a trainer. Well, not only did he get sound again, but quit spooking. But, only worked for a couple of years and came back at the next barn. Well, that one barn didn't have corn in their grain and the new one and the current one had corn in their grain. Took him getting worse with hundreds of hives, loss of hair, abscesses in feet and jaw, flared hooves, dry skin, atrophy and a potato sized dent in his neck, dull coat, tight muscles, balance issues, skin above his eyes swollen, I think that's all the symptoms to finally figure out his allergy last year. So long story short, another option to look into is your feed program specifically the grain. My horse had issues with Purina Strategy, Dynasty Senior, Purina Ultium, oats with a small amount of flaked corn and the worst Safe Choice Original. He is now on corn free Triple Crown Senior. He's now 16. Given your horse had a lameness issue if it was soft tissue you might be limited on the lunging. I was with my horse because he tore his deep flexor tendon in his front foot when he was four. The corn allergy did not help the issue either because of balance issues and the tight muscles. But, since he has been off of the corn, but vet had already said it has improved 50%. My vet had wanted to show his new intern the difference between the two legs with that injury and found the change. Good luck!