1985 Ford LTD

My LTD has a V6 engine. When I press on my brake pedal at traffic ligh stop, my check engine light comes on, and the engine runs rough and tends to die. However, when I press on the accelerator pedal, and the check engine light will go off and stays off throughout the highway speed. I adjusted the fast idle speed up some, and the engine run smoother at the traffic stop, and do not die out, but the check engine light still comes on until I accelerate, then it will go off again. I took it to the neighborhood tune up shop, and they told me I need to run a diagonostic test and brake diagonostic both to find out, and it will cost close to $150. Do I really need to run these tests to find out, or :P :P some of you have this experience and had it fixed. Thanks for your advices.

Stephen