I have a 1999 Pontiac Grand Am with 105,000 miles on it. I bought the car from a used car lot last year and they had to put a new engine in it. I hadn't changed the spark plugs on it until recently. My son-in-law replaced the spark plugs for me and now the #5 plug misfires. He can't figure out the problem. It also does not get a lot of power such as getting the car up to the speed limit to take it on the freeways. Also, the "Service Engine Soon" light stays on and will not go off. When I get to a stop light or sign, the light indicating that I need oil comes on and as soon as I go the light goes off. I know I don't need oil because I just got an oil change when my son-in-law changed the spark plugs.

Is it advisable to get a tune up on the car and what is causing the light to stay on? Any help will be appreciated. Thanks.