1994 Lincoln Mark VIII O2 sensor?

1994 Lincoln Mark VIII

This car has a system monitor display that tells you things like oil level, oil pressure, door ajar, coolant level, fuel level, and gas mileage, both averaged and instant mileage.
The 'check engine' light will occasionally come on, usually when getting onto the highway, but does come on at other times too.
When the 'check engine' light comes on, it won't stay on permanently, just turns on and then back off after about a minute so I don't know a way to have it checked for a code since the light turns back off.
I started noticing that the gas mileage read-out on the system monitor display will dip quite a bit during these minute-long periods when the CE light comes on. Happens sort of like this.

Getting on the highway and accelerate to top speed, and the CE light comes on. The gas mileage on the system monitor display begins decreasing from around 29 mpg down to around 21 mpg. After about a minute, the CE light goes off, and the mileage read-out increases back up to around 29 mpg.

I always thought this is caused by an O2 sensor failing temporarily. If so, which one would it be?
Any ideas?