New to site but long time reader

[lincolnls01]

I have been reading and referencing from the tech forum for a while,I bought a 2001 lincoln ls 3.9l v8,with 150k on the clock that made me a tad nervous,but ive had fords with much more. I feel in love with my ls after the first drive,the room,the comfort it fit like a glove,recently the car once put in gear,idle started to drop to 500-600rpms and and shaking,also shakes between acceleration from 40mph-50mph then stops again.if car is in park or below 40mph or above 50mph it does not do this,I read the tech article about c.o.ps failing.So I ordered my cops fixed them after a few cut knuckles,they were done, still have the same problem,I did an oil change at the same time,but still have the same problem the low idle while in drive and the shaking between 40mph till 50.

any help would be greatly appreciated as i already am arguing with the boss to keep it.

 

[lincolnls01]

also after driving for a bit it idles at 500rpms,i thought maybe cat was clogged but it still has power so im guessing a leak?