Mitsubishi Lancer 2010: Dangerous crawl mode

At the first set of traffic lights I come to my 2010 Mitsubishi Lancer CVT goes into a dangerous crawl mode, and the car barely moves. I have tried shifting to manual mode, but that makes no difference. It is too dangerous to turn off the engine. The dealer can find no problem and I get the attitude that I am imagining the problem. Having driven for 50 years I can work out that the car is not operating correctly. Sounds like a computer problem. How can I get this problem resolved?