Another straw to grasp - Might it be an anomaly in your intake manifold
that is changing the fuel distribution as the oil warms up? Something
like a gasket protrusion ever so slightly affecting liquid-vapor fuel
flow along the manifold walls (like the gasket between the carb & the
oil sump). A possible confirming symptom would be if the problem is
sensitive to fuel source & possibly if the power output "switchover" is
abrupt (then it would be someting else) or gradual over several
minutes.

The lifter pump-up on disassembly is certainly normal as you probably
now know.