Yes absolutely, I have a fresh set of plugs for this weekend and airfilter is new.Really hard to determine with out actually being there... but I would start with fuel. Possibly some bad gas, or a fuel blockage. Also, check the air cleaner, is it clogged.
Look at the spark plug wires, overall condition etc...
After all this, you may need to go to a dealer for a diagnostic check.
Ever change the Fuel Filter? I have a G & K Car too (a 2005 Passion I purchased new in 2006). I've learned a lot about ~450 Smarts because I had to.
It wasn't mentioned, but even though there is no check engine light, did you ever have what they call the "3 Bars of Death" showing where your gear indicator would display A for Automatic or a numeral from 1 to 6 displaying the manual mode? That can make one think the engine is running awful when it is actually an interruption between the engine and the transmission due to some wires shorting out because over time a harness of them was rubbing causing the insulation to expose the wires.
When that happened to me, I thought I was sure the engine was having problems. Yet all along it was what I just mentioned.