Allhumdulilah i just came out of teh same problem.
My car (City/Manual/04) was experincing teh same symptoms.
What i did was i went in for a routine tunning. Got the throttle body cleaned, and its plugs changed. When the mechanic tried to close everythign and do the "CNG setting", he found out that one of the coils was not working properly....i.e. the car was missing. thats why it was underpowered.
What he did wa, he checked the advancer of my car and found a problem with it. one of teh coils was burnt. He took the electrical thingie, opened teh advancer and soldered teh burnt coil with a new one.
The car engine came back to life like it was from a new car Allhumdulilah.
See if you can get this checked.
Below is teh detail of ruppees i paid.
Tappet Adjustment/Throttlebody Cleaning/:1000
Finding and Repair cost for my advancer: 350 rs
I think checking teh advancer will resolve your issue.