Hi there everyone

Nice to meet a friendly community of fellow dog lovers.
My story is about My Shih Tzu, Ace. I rescued him from a shelter about 4 years ago where his previous owners severely abused him. It took a lot of work and love, but I was able to get him out of his shell and he became a healthy and happy dog. Ace developed a severe case of Separation Anxiety (normal for a lot of rescue dogs), that while not able to completely cure, I was able to come up with compromises to create a happy household.
Now cut to a couple months ago. During a thunderstorm in the middle the night, I was woken up by Ace furiously scratching at the walls, panting heavily, urinating on the floor, and just generally freaking out. I was shocked at first as Ace has never shown any signs of anxiety with thunderstorms. For precautionary measures, I took Ace into the vet the next day for a checkup. Everything looked healthy. Unfortunately, the thunderstorms persisted for about 4 days, and each day Ace would freak out. Finally the thunderstorms passed and I thought things would return to normal and I would make preparations for when the next thunderstorm hit.
Well, then things got odd. Ace starting have panic attacks daily, with no outwardly visible trigger for the attack. The attacks would happen in the middle of the day, no thunder/rain in sight. Every single night Ace has a panic attack, tries to climb up everything, and was even caught once preparing to leap out a glass window! After about a week of this behavior, I brought Ace back to the vet, put him on ClomiCalm and another anti-anxiety drug (cant remember the name, begins with an "A"
;). While on the drugs, the freak outs are a bit more manageable, but they still happen several times a day, every day.
I've spoken with a couple of behaviorists, several vets, and done mountains of research, but I havent come across any examples of dogs with this type and severity of behavior. I dont like the idea of Ace on drugs for the rest of his life, and I miss my buddy and how happy he was. Has anyone out there heard or experienced anything like this? If so, what kind of treatment/training did you do? I appreciate any input you might have. Thanks.