P1450 EVAP Control code on 2000 Protege LX


2000 Protege LX 1.6L
The code readout from AutoZone says P1450 EVAP Control System (Excessive Vacuum). I've checked the gas cap and it seems ok. Nothing obviously broken or damaged around the charcoal canister, canister drain cut valve, hoses, etc.

Here's the wrinkle. The CEL goes off when there's a full tank of gas. It only returns when fuel goes below about 2/3 or so. Once the light comes on I've tried opening and reclosing the gas cap--to no effect.

I haven't noticed a dip in gas mileage. If anything, it might be slightly better. Thoughts on where I should start?