440 breaking up at 5600 RPM

A vacuum gauge might help sort out if it is a valve problem. The vacuum might get jumpy and decrease at limit.

Ignition current demand increases with RPM. I assume the power feed is direct to battery, with red wire enable to switched ignition. Ignition problems can be very RPM specific. Use timing light on one plug wire, see if light cuts out at limit.

Fuel related issues are often absent at no load, and show with load.