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Rough Running 1990 Eldorado -- GM experts please help Thanks

My 1990 Eldorado has developed a rough idle and acceleration, and I’m not sure why. When I purchased the car in July, I noticed a slight hesitation and a bit of a shake when I accelerated from a red light, full stop, etc. The car performed beautifully on the highway. I had my mechanic change all the plugs and give it a thorough tune up. He indicated that there was still a very slight shake at times from stop and that I should use some injector cleaner/treatment. I drove the car 100 miles yesterday and barely noticed the shake. The drive was a mix of highway and city - and the onboard computer listed 24.9 as the MPG. Today, on basically the same drive, I logged terrible fuel economy – in fact, my numbers continued to drop even at highway speed with cruise on. By the time I made it home, I was in the teens for MPG. I also noticed the rough idle was worse than ever before. I could place my foot on the brake when at a red light – the car would run relatively smooth – but if I took my foot off the brake, the car shook roughly --even without my other foot on the gas pedal. So, I have had a full tune up, I am treating my gas with injector cleaner — what could be the problem? Except for poor fuel economy, I would not have noticed a decline in performance at highway speed. This car has the 4.5 liter engine with 99K actual miles – and has been well take care of overall. Thanks for your help!