Thank you Malice.

I tried using your first function. The TEST_VAR has never gone above 1, so it does not appear that more than one instance of path_setter() is running at the same time.

Despite this, the monster is still having the same problem. Judging by the:

DEBUG_VAR(my.NODE_NEXT,500);

...NEXT_NODE keeps jittering between what appears to be 3 and 4, when the monster approaches the vicinity of node 4 to move back up path_001 .