I have a 1983 380 SEL and from time to time it sometimes while going on the highway doesnt want to go over 100 km/h with your foot to the floor! When this started happening there was also a hissing sound that developed under the car. I looked at it and thought it was the muffler or catylytic converter but if that was the problem it would do it all the time right?? I need help!

The car also makes a very slight pinging under hard acceleration but mechanic said not to worry. Is this OK?

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