Dave,
One of the problems that can sneak into the Fox35 NVA setup is air leaks around the case mount for the spraybar. I had a problem with a Fox that would start and set into a great 4-2-4 until the vertical 8 then start to lean out and go over. I tried all the usual suspects, plugs, fuel, tank leaks, etc. None worked, then by chance (better to be luck than good any day) I was there was light around the edge of the spraybar boss, I sealed this with high temp silicon sealer and boom problem solved. As the engine case got warm the air leak got bigger and leaned out the mixture. So seal both sides of the NVA mount.
Best, DennisT