2000 3.9 V8 coil/shake issu

I have a 2000 Lincoln LS with a 3.9L V8. My check engine light recently came on. It was a cylinder 1 misfiire and my mechanic replaced the 1st cylinder engine coil, 8 spark plugs, 8 boots and a valve cover gasket since a leak in it had caused some oil leakage. The day after the repairs my car again started to shake at idle and fight back at me when I reached around 45-50 MPH. My check engine light again came on and this time was a cylinder 2 misfire code and my mechanic replaced the coil in that cylinder. Driving my car after fix, I still noticed a shake during red lights and my car still breaks up at the same MPH (45-50). I brought the car back to the mechanic and he tested each coil and said that each passed. It has been about 10 days since the repairs and there is no check engine light but the same issues persist. Any insight would be greatly appreciated. Thank you.

Find someone with a scan tool that has the I/M Readiness feature. After driving the car awhile, get into the parameters and check for misfire counts on specific cylinders. If a cylinder is detecting misfires, but not over the threshhold, it will display those misfires, but not set a check engine light.

If the misfires occur enough that you are confident you will be able to duplicate the misfires, use the datastream mode of the scan tool and monitor the air/fuel ratio on the two banks to determine if the mixture is rich (indicating reduced spark or air restriction) or lean (indicating reduced fuel). A stuck fuel injector can also lead to an engine misfire.

-Rod