ASM hack: improved Gale Hairpin; please help

Your direct page register isn't zero (see C2/286E). What you might try doing is changing your LDA $11D6 to just LDA $D6.

It shouldn't mess with anything else you're doing, but if it does, you may need to zero the D register for the purposes of your custom function.

On second thought, that may not make a difference. It's worth a shot though, if for nothing else than added efficiency.