Pontiac Sunfire 98gt

I have a 98 sunfire gt, 2.4l, 150 000kms this is is the worst car I ever ownd.3 weeks ago the car started to idle a little funnie almost sounds like it wants to start stalling. Recentley ive done fuel filter, oil, fuel injjection + throtle body cleaning and the car is still acting up. It is also backfireing a little more than usual as it has been since I put new catalitic converter on it last summer. The engine check light came on yesterday briefly then went back out.I took it to the mechanic but luck would have it the car ran fine and they couldnt find a prob just old missfire code on diagnostic check jwhich may have been from when I did a new waterpump last summer and had to dissasemble+reassemble timming chain cover 10000 times to get funnie tensijoner to work properly then when I left it acted up again. It is only really noticeable in idle otherwise the engine sounds good. When it idles the engine shakes a little and sounds as if bogging out a little any ideas? Dont want to go to mechanic again if possible to check anything else on my own