After relaxation transient this spiral is stable for M = 1. Set M = 3 to watch influence of exitation wave alternation. For M = 5 this instability leads to the spiral breakup .
"Spiral breakup is found to occur when the minimum period below which a periodically stimulated tissue exhibits alternans in action potential duration, exceeds by a finite amount the spiral rotation period. At this point, oscillations in action potential duration are of sufficiently large amplitude to cause a spontaneous conduction block to form along the wavefront." 
 Alain Karma
Electrical alternans and spiral wave breakup in cardiac tissue
CHAOS 4, 461 (1994)