'Service engine soon' light comes on after driving 5-10 minutes

I have a 1992 Chevy S10 with 84,000K on it. It drives fine but when I get on the interstate and am driving 65mph for about 5-10 minutes the ‘service engine soon’ light comes on sometimes it takes longer. It only does it when I’m driving on the interstate and not if I’m just driving around town. It also doesnt turn on all the time but when it does come on it stays on until I turn the engine off.

You can retrieve codes using a paperclip. You’ll find the ALDL under the dash on the driver side. If my memory serves me right it’s a twelve cavity connector. 6x2. Jumper A and B together, turn ignition to run without starting engine. The check engine/service engine light will flash a code twelve three times- one flash, pause, two flashes, pause. it will flash 12 three times, the additional codes will flash three times as well. When it flashes 12 again after all that, it means you’re done. Make sure to write down the codes and post back with them. And I mean the codes, not someones interpretation of what they mean and maybe we can help.