Something In my #5 Cylinder?

I don't if it's bushed for floating rods, but if it is, I'm thinking bent wrist pin. At low rpm it's probably seized up enough to keep the bend situated downward, but at high rpm, the inertia gets high enough to spin the bushing in the rod bore, effectively lengthening the stroke. If it's pressed fit, it would probably involve the pin bore being cracked.