Thanks for your reply. I was afraid that no much people would know about the dual fuel system. It is used a lot in the Netherlands and in some countries in Europe. It is a liquid gas (something like where you cook on) You can build it in, in every car with a petrol fuel system, when you get the car from the factory or you can build it in at a later stage. In my car it was built in only 2 years ago. The disadvantage is that it get hotter than petrol and burned valves happens more than with petrol. It is to me however a mystery why it would run on one type of fuel and not on the other one.
Anyway back to your suggestion. The car it well maintained and the fuel filter is replaced very recently. I have also cleaned all injectors because that seemed to be a possible cause to me. The fuel pump is checked 2 times by 2 different garages and seems to be working fine.