Service engine light is on, off and on problems with ignition startup, when idling or slowing down car spasms and rumbles. Feels like it may break down at any time. Diagnosis?

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The "service engine soon" light has been going on and off for several weeks. Problems have been worsening off and on yet when I take it to the mechanic, they can't find anything wrong. My car almost broke down 3 times today. The problem has been off and on, and today much worse. When I slow down or come to a stop there is a lot of chugging and shaking and after slowing or stopping, when stepping on the gas pedal it is having trouble functioning and the car starts to rumble. It is getting very stressful that the mechanic I have trusted said he can't fix it until the problem is happening at that moment. I feel like it's going to take breaking down on the parkway before they can tell me what's wrong. Any ideas? Thank you so much for your help. I need to get this fixed tomorrow. Don't want to wait any longer since it keeps worsening and I don't want to brin Oh I almost forgot. And sometimes the problem starts when I first turn on the ignition, like the ignition won't fully fire. Then the engine rumbles a bit before I get the car driving and it smooths out for a while. I recently had a new ignition coil put in, but the mechanic said everything checks out ok when I've asked to see if the part may be defective.