I've had a couple of PS3 steamers do this. It's a problem with the tender connection at the wireless drawbar. My missed chuffs were mostly occurring when rounding a curve. One I fixed it by opening the tender and swapping the 6 pin wire connector from the tender board to the post connector where the drawbar connects. I cut the zip tie on that wire connection and left it off to give the swivel on the tender a little more free motion. Another had a bad solder on the wireless drawbar.
Everything else worked great on these two engines except missing the occasional chuff sound (when the chuff missed the smoke unit also didn't puff).
