Cadillac El Dorado

I have a 1984 Eldorado with an HT4100 engine and 33400 miles on it. Whenever I drive on the highway for about 10 minutes running at around 65 mph, upon stopping my engine starts to diesel, ping and knock and would die out if I did a full stop, but I would usually slow down hit the brakes and let go and wait for it to stop dieseling and stop again and it would go to a normal idle, but then when I will step on the gas to proceed, the engine will start diseling again and the car has no power. I noticed that after I park the car and drive it again after about 15 minute everything goes back to normal. If I drive locally with the stop and go traffic, the engine is normal. It is only when I drive on the highway at high speed and slow or stop is when it start dieseling again. Thanks for the help in advance.
January 10, 2007.

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