Yup, too little info given. Please send to mechanic for check up.
To me probably low idling speed, because when step on accelerator you mention okay, right?

Was that the same mechanic who ask you to change the fuel pump??

Just hate those mechanic trial and error by changing parts and cannot locate the problem.....

One set normal plugs only around RM20.....

Try checking the spare part shops.....

You mean to reset the ECU?

Probably, but no need overnight, just a while will do......

Since the only thing you change were the plugs, then probably was the plugs......

Not piston gone bust. Piston kaput cannot start even......

Some ignition timing out, plug wire leak......

If noisy then needs adjustment.....

Tappet if loose, idling also have sound. If tekan got sound maybe knocking already......

Problem is, it is intermediate, right? Sometimes okay, sometimes knocking. Like that really need to leave there at the SC for check up.....

This is normal, happen to me a few times also. Most of the time I am able to diagnosis the problem, so I will just tell my mechanic, but of course at times there are special cases that I don't know what is wrong also, and every time I reach mechanic shop everything is okay.
I remember this case when the car has warmed up there is a creaking sound coming from the right front wheel, so mechanic, take out wheel to check, jack up with wheel on, jack up whole car to check under carriage but still cannot find the problem. Problem is once it cools down slightly no more sound.
So got one time when the sound came, I drove all the way to the mechanic, and ask him to check immediately, so he jack up the whole car. This time he had his workers to move the car up and down as like going over the bumps that creates the sound. He finally found the sound coming from (you will never guess) a dried up tie rod end but the left side one, not the right side one. The sound channeled itself to sound like it came from the right......

Yup, it is true. He had a new customer who was introduces by one of his customer. The new customer have some jerking problem when driving and mentioned that previous mechanic he sent to had changed the spark plugs, the plug wires, the distributor cover and a whole lot of things already and still not solved. So he got fed up paying for all the things changed but problem not solved.
My mechanic after checking his car called me up because I happen to own a similar car and needed to try out the ignition pack. So I drove over and he use my car parts as guinea piggy . Yup, the first thing he transferred over and the car was running fine, so he just ordered that part and changed it. The new customer was so happy, so became his long time customer now....

Let them try out first. SC have all the equipment, but not sure of the personnel only.....

My mechanic is in Puchong.....

Could be an intermittent problem with your wiring, but have you asked them what happens if it is not the wiring harness problem?? Change for nothing? Not cheap woh!

I think Myvi is direct coil over spark plug, right? If that is the case, then no distributor.....

SC trial and error also, because don't know what is wrong?

Because like you say, they already change almost everything......