Brian, if you're at Medford, we can talk then. You're not running anything funny in the fuel, are you? Probably best not to answer here that you've got 90% nitro, but what the heck, I haven't seen any tech done in years.
Like G said, you've got a valve seating or a valve bouncing problem. Those big stock valves are heavy and will wear out e set of doublr springs quicker than the stainless valves George and I use. Also, set your lash at .004 COLD. No closer than that.
Bert