Plugs might just need replacing. Mine “hiccups” every so often, and it will only get worse. Last time I just changed out the plugs, and it has run great for a while and now it is coming back slowly. Time to change them again. I’m using NGK Coppers and they only seem to last for about 6k. Mike used to change his every 3k.
edit:my plugs last change came out looking fine, and weren’t fouled either.