89 LE. SES coming on intermittently, more commonly downhill. Does not affect performance or mileage so far. Been happening for the past 50k miles. Car now has 392k. Storing no codes.

Hmmm. Well my Cutlass Ciera used to do the same exact thing. It once stalled at the bottom of a hill after doing it...and wouldn't run. Then for no reason it started up and I SCOOTED it up the other hill WOT

So no advisory?

Intermitten lights could be caused by bad electrical connections, battery, grounds, or sensors. Try cleaning them first, then check your TPS voltage at idle position. Going down hill it is in this position and if the voltage is off, it will effect timing and fuel setting a light temporarily.

Dont know if this helps but my SES light would turn on and off periodically with no codes, later i discover i could make it do this if i was slowing down and very lightly applied the brakes. It go away after 15-20 minutes.

I guessed it had something to do with the EGR valve not lowering the rpm enough, since it engadges with brakes on. (i could be wrong)