Engine Service Light = hard transmission shifting

I have a 1996 Chevy Suburban K1500 with 85,000 miles. The car is incredibly well maintained with service records complete. The engine service light comes on after continuous driving for more than 45 miles and sometimes as much as 200. As soon as the light comes on the automatic transmission shifts hard in the lower gears( 1-2) noticeable in (2-3) but not going down in gears. After the trip say the next day the light is off and the shifting is regular and smooth. I have had it look at but of course the engine service lite is not on and the mechanics say everything is fine. The transmission never shifts hard without that light present.