pipe popping

You can always do the process of elimination thing, remove #1 spark plug wire and test drive. No change, hook #1 back up and pull #3 etc... until something changes. A compression and/or leakdown test may help, if only to confirm the engine is healthy.

Sticking valves and leaky valves generally get worse as the engine heats up, and broken valve springs are more RPM related. However exhaust leaks may seal up as things heat. I think that's your problem. Replacing header gaskets is fun, so start there. Also look for tiny soot trails at the collectors, and any other exhaust connections.