Check Engine Light: 87' 305 TBI, R10/C10

[Fan2zee]

I did a thread search and couldn't seem to figure it out.

I have an all OEM setup: 1987 305 TBI

My check engine light comes on for a minute or two, then goes off. It never stays on long enough, so by the time I pull into the garage it's gone. The light only comes on at highway speeds above 45mph. I have an OBD tool, but it shows nothing in the history alarms so I'm having trouble figuring it out.

Everything seems okay, the drive feals great, all fluid levels are ok, gas caps seem tight, I've ran several tanks of non-ethanol & 93 octane, same random alarm, all gauge readings are normal.....

Maybe O2 sensor going out?

Thanks