In the last year, I know of three people who have had customers cars in their shops to address the exact same problem.

In those three cases, the cam had been installed advanced by 2.5 teeth.
They had the bottom gear key way straight up, dot on upper gear straight down........ instead of “dot to dot”, which will have the bottom key way parallel with the #1 cylinder(not straight up).
The cam installed key way to dot is advanced 35degrees.

It sounds like some kind of valvetrain issue to me........ cam timing, springs, lifter preload, etc.