Yes this model of the mehran has this issue. I got myself a zero meter vxr euro 2 in September 2015. This issue came up with me after around 6 months. I got it checked from madni motors lahore. The guy said that whenever it starts missing, just open the fuel cap and release the gas built up in the tank. Dont ever take it to a local mechanic for this issue as he'll rob u by changing plugs, etc. This resolved my issue but on a long route, u need to do it after every 70km.
Before this model, i had a 2000 mehran plus model. Now that model had a plastic pipe just behind the left brake light, coming from the fuel tank. That pipe released all such vapours and gases from the tank (which is why Suzuki Mehran always smelled of petrol). If somehow that pipe be fitted in this model, issue might get resolved.
Somebody in this thread also suggested buying a cheaper fuel lid. Good idea. Just remove that rubber seal to avoid vacuum building up in the tank.
Hope it works