This is usually indicative of the hold winding on the flipper coil being open circuit. The flipper coil has two windings; a short thicker wire (for high power) used for thrusting the flipper, and a long thin wire (low power) for holding the flipper in the up position.
When you press the flipper button and the flipper thrusts, once it reaches the end of travel and opens the end of stroke switch it relies on the hold winding to hold the flipper up. If the hold winding has a break the flipper falls back until the end of stroke switch closes causing it to thrust again. This results in the fluttering.
On the coil itself, check each of the thin winding wires that runs to two lugs on the coil to see if one is has a break.