Right now I can think of thousands of things that can go wrong. Genuine plug wires are usually good for 80,000km (standard replacement interval). And nowdays plugs also last too long. Don't know about the 4 cyl G10B, but the standard NGK BP6ES/BP6EY of 3 cyl cultus have a standard replacement interval of 10,000km. I have run them upto 25,000km before they start missing horribly. The occasional missing starts occuring after 18,000km. What I want to tell is that spark plugs live wayyyy longer than designated. I did this experiment on an F8B with contact-point ignition, which is lower voltage than CDi (that is standard in G-series engines). CDi has higher voltage so you should be able to run them even more (high voltage=low resistance). The standard NGK BK6RE in Baleno G13B (which I hope is also standard for G10B) has a replacement interval of 40,000km. Now you can check your spark plug model number and find out how long had you used them before somebody told you to prematurely replace them.
One thing I came to learn is that when a problem is small it is difficult to locate. Don't waste lots of money. Instead, develop a driving pattern in which you don't continuously drive at 2.5x100 in 2nd gear. Soon (in 2,000-20,000km) the problem will become big enough and you'll be able to replace the correct parts. Also, you'll be amazed at your resilience and adaptability.
Are you running a very rich mixture?
