About your rpm not stable, try to block the air intake by almost 40%, set the peak screw from CNG to its optimum, set the idle according to what you think is ok, the car should not stuter!.. after that, take the -ve terminal of the battery out and then put it in after 3 mins and restart the car.
IF the problem is not solved, just get your throttle body cleaned,
I just bought a vitz, and you can see my posts from page 180-uptil now. The reason i'm telling u is that once i got the vitz, i got it tuned, performance increased, i followed the instructions on this Wonderful FORUM and got all the work done(which i wouldn't have, it wasn't necessary) but like vitzrules says, maintanance. After it, its all perfect and i'm having pleasure driving my car. So this is the only forum wherre you can get free and honest advice of vitz users. So Join in, get the car , get it inspected though:D