2002 Mercury Sable rough idle with misses

I recentley had a good friend and great mechanic work on my 2002 Sable. He generally dose not work on Ford products but he made the exception. I was having a terrible rough engine idle, with many missfires. I took it to have it coded at Auto Zone. The readout was that I had a problem(missfire on cylender #1). It also Inidcated that the cam shaft positioner may have a problem. I was able to replace the cam shaft positioner and fuel filter myself. I did have my mechanic, take it from there. He replaced the wire set & plugs. He also found a leak in the thermostat housing unit, and replace that as well. However after spending $450.00 I still have the miss in idle, but now it is worse. The engine starts out rough in drive, but does smooth out after getting through the gears. Then, while in overdrive if I try to accelerate, it hesitates & chugs. I have to back off the throttle, then it smooths out again. My mileage has drop substantially. can you make any suggestions to help me pick a direction to take the proper action without costing a bundle. I love it for my work car, but is it worth sinking any more money into it?

Thanks.
Bobby Beardman