Possibly a bad / leaking brake booster basically creating a major vacuum leak when the brake pedal is depressed. Had one in the shop just last week doing exactly the same thing & that was the problem.
Testimonial: "This was a helpful lead...THANK YOU! I'll go check for that vacuum leak. I've changed the plugs, replaced the o2 sensors, now I'll check for leaks."
You could try pinching closed the vacuum hose to the brake booster wih a pair of vise grip pliers or otherwise and see if the problem still exists. If it still does it then that is not the problem. Dont drive the car with the hose pinched though, you will have no boost and stopping could be extremely difficult.
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THANK YOU AGAIN! You were right on. We found the vacuum leak and now the car runs LIKE NEW! THANK YOU for sharing your wisdom!
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