Start loving your car & it will love you in return no matter what condition it has, that is what I have learned after changing 3 cars....
If you get it on your nerves & start thinking k is ne boht tang kia hai to it will remain sick for sure no matter how much money you spend on it, that's my personal experience although I don't know the reason behind this phenomenon
Now coming to your problems. You said Head gasket is OK, so I relying on that & moving forward:
1- Change your CNG/Patrol Pump at the first place.
2- Get your complete CNG kit serviced including the cylinder cleaning as well from a renown workshop
3- Get your car tuned-up including tappet adjustment but from a good mechanic, not a road side chief engineer.
4- I don't know how to explain but as commonly known, engine ki timing adjust krwao... I think that is the problematic area, as same problem haunted my Margalla-96 for a long time until I got it's timing adjusted.
5- Try to learn & understand your car & start dictating the illiterate mechanics about what to do & WHAT NOT TO DO instead of asking them to figure out the problem... They will not do any good themselves for sure unless you yourself know a bit.
This all won't cost you much. I remember a mechanic asking me 10K for timing adjustment of my margalla & another one around 3k & finally my current rather permanent mechanic did that for free without telling my that what he is going to do during the tuning & then I came to know what actually the job was... :P. So the point is that things are not actually that complex but your approach towards the problem & the people who guide you actually determine whether you will sort out the things or you are going to surrender soon.
Best of luck!!!