Li,

AT does not mean 'overtone resonating' or anything like that. It's only a description how crystal is cut from original quartz, means it's orientation regarding to axis of original quartz. Beside AT-cut also CT-cut and DT-cut exist. But only AT-cut is able to resonate beyond 1MHz. So, whether your 16MHz quartz is resonating on fundamental or third harmonic, it will always be an AT-cut crystal.

I haven't read datasheet of your chip, but isn't 18pF a bit low? Try 27...33pF.
Try also a new crystal, and a new chip.

Kai