M6 Remap Issues - Weird Queue Buster Problem

I currently have a custom M6 remap that intermittently gets stuck on a line with an M66 (M66 P7 L3 Q10

I am assuming it's an issue with queue busters in my gcode, and I have a rough understanding of how queue busters work, and why they can cause problems, but I would have thought an M66 is exactly one of the ways to prevent a remapped code or gcode program from getting stuck, as explained here:
linuxcnc.org/docs/html/remap/remap.html#..._position_prediction

The tool change also gets stuck occasionally within the second half of an if statement on a G53 G1 move. 

Any general advice around queue busters (or other causes) for a tool change to get intermittently stuck is appreciated!

Edited for clarity

Getting stuck on a G1 can often be related to spindle-at-speed being false. How is that driven?

The issue indeed did turn out to be related to spindle-at-speed.

Was able to fix it by ensuring spindle-at-speed was set true during tool changes.