my '03 SLK 230 (90,000 miles) is doing wierd stuff--after 15-20 minutes of 70 MPH on cruise control, the RPM's will start to surge up and down by about 20+ RPM. This suring continues while in normal city stop and go traffic. After shutting down the engine, every thing seems ok until another run with cruise control. I've tried the soft ECU reset--still happens after a cruise control run. Ideas??