Diagnosing cylinder misfire

Did you set the preload on the lifters. Sounds to me like the lifters pumped up and hold the valves open with oil pressure when running. Sometimes you won't pick that up on a compression test. Stamped steel rockers are noted for this unless you shim the shafts correctly. Just a thought. A vacuum gauge may picked that up watch the gauge as it warms up.

Also check the left side muffler . I have see varmints move in when the car is sitting.