2004.5 S40 Rough idle, flashing check engine light

[noarch]

Hi folks...I'm new here, but I (and my mechanic) are stumped.

I have a 2004.5 S40 with about 75,000 miles on it. Long story below....

About 2 years ago, I had a check engine light come on and a rough idle, especially after coming off from highway driving. Drive on the highway for a while and return to city streets and my idle would be rocky, and I'd lose power and the car would rev high before shifting when pulling up hills (steep hills here in San Francisco). Mechanic said bad turbo sensor. Replaced it, and that seemed to solve the problem.

A few months ago, the same thing started to happen again -- exact same symptoms. Took it to the same mechanic who said that this time the oil trap needed to be replaced. I needed to wait a few weeks before I wanted to pay for that repair. Took it in after a few weeks, and the mechanic said not only did the oil trap need to be replaced, but now the car was also giving him codes for two misfiring cylinders and that those cylinder coils needed to be replaced.

I picked the car up, and it still seemed a little rough, which he said was probably just clogged injectors, gave me some cleaner, and said it would improve. A few days later.....driving on the freeway, pull off onto city streets and the rough idle starts again at a red light, and this time the check engine light starts to flash. Take it back to him, he says the misfire codes are showing up again, he replaces all 5 cylinder coils and all new spark plugs (at no cost after I gave him a piece of my mind). Seems to solve the problem, but still running a little rough.

Two weeks later, here we go again. 25 miles on the freeway, get into the city at a red light and the light flashes, I lose power trying to get up a hill, and the idle feels like the car's ready to stall out when I'm at a red light.

I'm lost. My mechanic seems lost. He's done a smoke test to confirm there aren't any air leaks, and he's done a compression test, both of which come out just fine.

What else could it be? Catalytic converter? Fuel injectors? Head gasket? I'm just about ready to push the car into the San Francisco Bay.

Maybe some of you will have some ideas that can point us in a better direction??

Thanks, and thanks for reading!