Is the pop from the intake or exhaust? Either way, a pop under load is coming from a sticky valve, or one that's beginning to burn a little. Since it's only happening with the engine fully warmed up, I'm betting on carbon deposits on one or more valve stems. The only effective fix is a valve job, but you might do a temporary fix by using a lightweight oil additive like Marvel Mystery Oil or KROIL to the crankcase. Use half a pint of MML, or about 4 ounces of KROIL to an oil change. The oldtime fix for a sticky valve was Casite Tune-Up added to the oil, but I'm not sure if that's even being made any more. If it is, it's probably a poor imitation of what was available 40 years ago.
Jerry

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