Problem with up hill acceleration and higher speed acceleration. Car loses power in middle of driving.

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I had the spark plugs, spark plug wires, valve gasket replaced. Still having a problem. There was oil all over spark plug boots because of valve gasket leak, put new spark plugs in before leak was fixed but my mechanic fixed the gasket and wires and didn't replace spark plugs because they were new. Still having a problem. I can start up a hill fine but at a certain point it just bogs down and won't accelerated, i have to take my foot on and off the gas to get the car going again. Same thing when I am driving at 65 mph. It will be fine then suddenly drop speed. Should I just replace the spark plugs again? I don't think that is the issue. Also the check engine light comes on when it happens and then turns off. Otherwise the car runs very well it only has 160k miles on it. I have had it for 6 years. Also I had this problem a while back then it stopped after cleaning a small piece of foam in the air filter system. I don't think that's what stopped it because I did that again and it didn't change this time. It actually started again when I had the struts replaced and got a regular oil change. I let a broken strut spring go for a long time so the rear end of the car was sitting low for a long time and now it sits up right. Don't know if that makes a difference but thought I would give all details.