Two weeks ago I was stopped at a traffic light and my engine shut down. It took 4 tries to get it back running. People behind me got pissed when they missed their green light. 

This morning the same thing happened. I was waiting for a traffic light and a ton of cars when the engine stopped. The same thing happened when I tried to start it. This time the engine shut down on 4 different occasions and it took 2-4 tries to start it on every one of those occasions. 

I found [a website](http://www.go-lpg.co.uk/Hints.html) explaining that if the engine stops on gas, some gas will remain in the engine. When starting again, the mixture of gas and petrol will be difficult to ignite, so that could explain the difficulty in starting after the initial shutdown. 

What could be the cause of the initial shutdowns? I would love to be able to solve this one myself. If there is more info needed I'll provide it, but I'm clueless about what you would need to know. 

My car is a Volvo 850 2.5 from 1996 with 550k+ km and I don't know how long this vehicle has been running on gas. The car always starts on petrol and switches after a second or 2. It doesn't seem like I can prevent it from automatically tyring to switch to gas.