2000 kia sportage engine issues

I hope someone can help me.
I know the 2000 sportage has overheating issues however I have a new radiator a new engine fan a new water pump a new thermostat now i have a blown a head gaskett and replaced that and the oxygen sensors before and after the cat convertor the check engine light is still on and the mechanic is stumped what next the issue its having lately is misfire and popping when in idle at lights or stop signs once you get going its ok other than the check engine light please help

Reading the codes would provide some clues..... ignition problems , specially coil packs - are not uncommon ..as well as sensor connectors; mine isdoing tricks of that kind now...but I know it is a little oil spill on #4 plug and/or a coil pack that had already developped an ark path so will open that when winter is over so I do not drive a lot now
...........but, come back with the codes first.

Thank you for the reply. The mechanic replaced the sparkplugs yesterday and it seems to be fine now. However I'm not so sure it could be that simple because nothing has been with this kia so far so, when you say the codes are theese what you get when having run on a diagonstics machine and can you have it done anywhere for free. This latest adventure was 1200.00 so I'm getting to the point of no money left to sink into it. Thanks again

1200$ for a head gasket job is within the limits, though in the high end of the range for that engine. It depends on what they might have changed at the same time (timing belt, coolant pump??) in which case it would be a good preventive maintenance at same time.
It is told that Autozone pulls the codes for free (i do not live in the USA). If you go for that, write down the code numbers, not the meaning they may tell about them. But yes, it can be as simple as new spark plugs, May be once they were there, they have done some cleanup around them too: oil spill is not uncommon..but if you have a new head gasket installed , then obviously they must also have installed a new valve cover gasket that could have caused some oil spill on spark plugs and plug wires in the past....
....hence if all is OK now, you can wait and see....if the CEL is still ON, after a few engine cycle with no errors, it will fade out; if not, always time to go for the codes.

Thank you so so much for your help