[Certimafied]

I would guess lean mixture as Chuck Sea/Tac said or timing is off for that engine rpm and load.
I recently had a Porsche ( I know way different beast hear me out though) that had some surging and backfiring problems at cruising speed . I ended up finding the intake valves way too tight and a series of small vacuum leaks. Adjusting the valves cleared up a lot of issue and fixing the vacuum leaks cleared up the rest.