Hey. My horse is a pacer as well. I've tried almost everything. The only thing that really has helped was putting her on quiessence, maximum turnout, and a slow feed haynet. Also if she starts pacing real bad, I tie her. After a little bit I untie her. She's alot better then she was. Around feed time and turnout time is when zshe does it now mostly. But not nearly as bad. Hope this helps.
The symptoms are quite specific... the horse turns tirelessly in circles in his box and the straw is found in piles in the middle of the box. So that your horse stops doing this, you have to take it out more often, open the box window and let him see other horses.
It is also important to review the feeding of the horse which must be too rich and its program of work which it must not be complete enough. Passage to non-edible litter is also a risk of triggering this behavior.
Stall walking is mainly due to boredom so maybe you could try putting a lick up or a puzzle feeder type thing. Like the balls that they push around to get a bit of food out of. Some people also use swedes on a string as it keeps them occupied and doesn't add sugar to their diet.