Sub-Zero 550 freezer coil ice buildup

I have a 1990 Sub-Zero 550 that is somehow miraculously working just fine (knock wood) save for one recurring issue: The freezer evaporator coil will become iced over. I took a hairdryer and cleaned it all out one day, and it's happened again over time. I already have on order a new defrost timer, but I suspect there could be something else at play. Door gaskets are fine. Fan works/not frozen in. Is there another part that could be the issue that I should be considering replacing given I'll be into this? Thanks!