Chevy Overdrive troubles

2000 pickup does not seem to shift into OD all the time and is running in 3rd gear at times on the highway. This is a 4 dr model with a 5.7ltr engine.
Is this an electrical problem?

Could be lots of things. How fast are you going? Going uphill? Is the engine up to temp when it happens? Is your left foot touching the brake?

I have the same problem with my 2000 3500 crew cab with a 7.4L. It will run fine till it get warm up then if you go up a small hill in the road it will down shift into 3rd. It will stay there until you put it in neutral and turn the key off and wait for all the dash gauges to quit then start it and it will run fine for about an hour. Then it will do it again. Any help?