This sounds exactly like what I'm having issues with, Las night we played around for a while turned it up to 10-11 ran great turned it up to 13 and bam falls on its face missing and sputtering.
So we went back to 10-11 psi and it's fine again runs clean, plugs are tr6 gapped at .034

 

Time to regap the plugs...I ran the tr6's around.024 with 18psi. I gas fouled those and put my old br7ef's back in at .024.

 

Great thanks