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Exhaust back pressure/valve float

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Has anyone ran into this before on a car?

How can you say for certain that's the issue without exhaust pressure gauge?

It's not something I've personally experienced but if the conditions were correct (or very incorrect as the case may be) then it is quite possible.

A give away on an engine equipped with hydraulic lifters would be the engine dropping one or more cylinders - As the valves float, the lifter will pump up or extend to take up the lash clearance between the follower/bucket and the cam. This holds the valve open and the engine drops power. Normally when you return to idle, the engine will run poorly for 20-30 seconds as the lifter bleeds back down and the valves start to seal again.