99 Metro service engine soon light/shuddering

Okay all, I'm new to the board and only slightly familiar with auto mechanics. Hubby has mechanical knowledge, dad is a lifetime master mech, but not much time to help with the car without knowing the problem. I have had the car less than 6 months and about every 3 months (oil change time) the service engine soon light has come on. There is a shuddering effect when you go to start moving from a stop. Have checked all fluids, spark plugs and added injector cleaner. Didn't help any. Last time it did this (less than 3 months ago) I took it in to the shop to have a diagnostic and oil change done. They told me it was only fouled spark plugs causing the problem and changed them (the light went away. Figured ok problem solved). They charged me 250 dollars for this, which I think was a rip-off. I could have done it myself for 10 bucks if that's all it was. This time though. The plugs are fine, fluid levels are fine. Still shuddering/stuttering pulling out of a stop. Moreso when the engine is not fully warmed up and have heard a slight ticking sound (like a lifter tick, but oil is ok. Maybe exhaust?). Any thoughts that could save me a few hundred dollars on another diagnostic? :( Should I ask them to do another one for free since the problem didn't get fixed correctly last time? HELP