German, I assume you have a carb on yours. If so, try adjusting the automatic choke first. Turn the black plastic choke cover to align the marks; one is on the cover, the other is on the housing. This is a starting point, you can fine tune from there. Also make sure the choke linkage is free and lubricated. Make sure the choke plate moves as you move the choke cover. If it does not, the bi-metal choke spring may be broken, out of place, or sticking.
The second thing to do would be to make sure the idle mixture screws are adjusted properly. To do this, turn them both in until they bottom (counting the turns for reference). Then turn them back out until you get the fastest idle, about 2 and 1/2 ..........3 and 1/2 turns. The best way to do this is with a vacuum gauge. You just turn them out until you get the highest manifold vacuum.
I agree with LoneRanger. It seems that your truck has a problem in the carb, and possibly in the carb emmission system. The truck runs because the choke is holding the carb plates open during idle, but once you press on the accelerator you trip the choke mechanism which lowers the idle too much for conditions. I can't see the engine, but at least have it checked out for loose, missing, cracked vacuum hoses also. If all of your emmission parts are in good order the solution to your problem could be just a simple adjustment; or may be a replacement of the choke, and linkage assembly.
I wish you well with your problem. I wonder would you mind answering a question concerning your unit. Is a full size Bronco popular in Germany?. I had never seen one of these unit used overseas (with respect to the US). Are there sources of parts similiar to BGW, and Audi in the US?. Have you ever posted pictures of your unit on-line. It would be interesting to see a Bronco with a right side steering wheel, e.t.c
Well good luck whith your unit.