There is a mechanic in Lahore at Hakim Chok, Called Haji Sahib, The guy has resolved the problem of many Vitz. The issue is niether of tunning nor the brakes. He will remove the Venturi and Install an injector in the throttle body, few more adjustments and u will never have any problems with the Vitz on CNG.
aoa bro, hope u doing gud, bro ii have same problem in my car , i saw ur thread where u mentioned a mechanic Haji Sab in hakim chowk , can u please guide me more abt its location or phone number so that i can resolve issue. my email is email@example.com