Electrical problem in the car

When I'm idle (at a stop sign/light) my cars RPM jump around between 500 and 750. I took it to a transmission place, they did a test and said it was an electrical problem, but they couldn't tell if the computer was shot or if it was just a wiring problem. Any suggestions here? Wouldn't a computer problem mean all the electronics in the car would be struggling? If so, the radio, AC, heat etc etc all work fine.

Other info: The car is a 2003 Mazda 6, 4 cylinder engine, about 113,000 miles. I get regular work done. The problem started happening after about a 6 hour drive, once I finally slowed down from highway speeds and was waiting at a stop light it happened.

have you checked your egr valve? vac leaks? both of those will cause the idle to jump and neither will cause a cel