i thought cuore comes with CDI sys, anyways, check electrical system first,i.e. impedance of plug wires, plugs, coil, timing etc
check timing not only on idling but with higher RPM, make sure TAP is doing its job on petrol as well as on CNG. try by passing it for some time and then check it again.
if everything goes normal, then probably needs to check flywheel, clutch plate, pressure plate setup. maybe not faced very well.
btw our baleno got the same problem, and suzuki guys pointed out a degraded performance of camshaft making timing out every now and then but its some 180K driven.