Misfire number 8 cylinder disappears when timing set at 18 degrees, yet reappears at 5 degrees

[Sierra 1998]

Any ideas why a check engine light flashing (misfire code) for number 8 cylinder, disappears when timing is set at 18 degrees advance and reappears when timing set at 5 degrees advanced, by my GMC trained Mechanic?

Engine is a replacement, GM crate engine with about 20,000km on it. I changed my engine when it died after a flashing check engine light came on then engine died after a 10km drive, necessary drive as was -30 celsius and no traffic on highway. New plugs, wires, distributer cap, coil installed. Once engine was installed, test drive had no codes, but once delivered misfire code started. Found loose distributer, tightened it down with a rough guess on timing, check engine flashing disappeared. Took into GM Dealer to check timing, they found it advanced 18 degrees, so they move distributer one tooth and timed to 5 degrees advanced. Flashing check engine with code for misfire in number 8 cylinder came back. Mechanic is thinking tippet is wore, is this possible when misfire code disappears when timing is advanced to 18 degrees, or is there something else?