About 2 weeks ago car started hesitating intermittantly, slower acceleration going uphill. The check engine light has come off and on during this time. Very recently the check engine light has come on and stayed on. Car is now very sluggish going uphill. Depending on the incline and length of hill car speed can be as low as 20 mph. Even with the cruise control set, the pedal will be pushed to the floor and car will continue to decelerate. RPM's go higher and there is a smell almost like sulphur. However, when driving on a flat surface car seems to drive normally, acceleration is normal, speed is normal.