Its common problem with Alto. But in ur case 100 on petrol is too bad....It should be atleast 110 to 120 on CNG and 130 to 140 on Petrol.
Please check if company fitted CNG that venturi is working fine and air intake is not half blocked. If venturi is okay and air intake is blocked then revert it to its original. Sometimes venturi is not working properly, then u have to half block the air intake.
If not company fitted CNG then ensure that air intake is half blocked and car is tuned well.
this is just u have to confirm.
The actual reason of such behavior can be plugs gap issue. As your car is not too much old and also u have overhauled ur engine recently, there shouldn't any issue with engine.
Adjust plugs gap through a good CNG mechanic and if your plugs are old then change the plugs.
Another possible issue can be distributor/coil but if ur car is not giving missing problem and stable at idle then distributor is okay.
I typed very quickly, sorry for any mistake. All the best.