That’s frustrating , are you running the Motorcraft plugs that came with the kit ? I was having some random misfiring so changed plugs to NGK seemed to take care of it .
In your picture the silver colored idler right before the tensioner . Remove the belt then take out the Allen head bolt that holds that idler on and see what spacer/ boss is behind it should look like this one.
I had this exact same issue . The problem with mine was the kit was missing one idler spacer . The shop that installed mine made a spacer to get me my car back . The problem ,the shop made spacer held the idler out to far and caused the belt to come off the back of the idler then walk one cog...