check engine light is on on 2001 Cadillac DeVille

When I press my brake the car shimmeys and when I slowed down to 30 25 mph it started lightly jerking. Than it went away. I drove today at 55mph and it started to jerk again. My check engine light has been on for this past week, I got it checked at a local mechanic he hooked it up to a machine but couldn't find anything wrong. What's going on with my car. He turned the check engine light off twice but it came on again when taking the car for a test drive. Help!

by in Owyhee, NV on March 23, 2010
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2 answers
ANSWER by , March 25, 2010
If the check engine light is on, there is a code stored in the engine control module, and the "mechanic" seems to be erasing the code to turn the light off but not diagnosing what is wrong. Can you post the diagnostic codes that the mechanic pulled from the control module?
ANSWER by , April 15, 2010
engine light on. 20miles tothe gal. car works great. took to dealer 1860 now I have a bill of 3300.00?????????????.