I have a '96 tahoe. Sometimes when I use the air conditioner after about 50-100 miles the ‘service engine soon’ light comes on and the rpm’s move to 2000 going at 65 mph, normally they run just 3 notches below the 2000 mark, then when i stop at a stop light or so and start off again, the kick out gear slips or makes a loud noise. After about a half hour or so, when i restart it, it shifts normally. I have asked almost everyone, and nobody really knows.
Go to Autozone when the light is on and have them scan the codes.
Without more information, I think the problem could be a bad ground or intermittent short circuit, or maybe a loose connection in the wiring.