340 shuddering/cutting out/jerking

Hi, it was only a few months ago when I had the same problem on my 340.
I don't think its an electrical problem, there would be other symptoms as well. It's probably a fuel problem, not enough fuel in the engine. I would check the jets in de floatchamber and the fuel hoses.
I don't know what type of carburettor there is on your car, but I think this procedure is valid for most carburettors.
Remove the top of the carburettor. Be careful when you lift the top of, some petrol might drip out and there is a gasket between the top and the body. You can reuse the gasket. When you look into the carburettor from the top you will be able to see the float chamber. On the bottom of the float chamber there are 2 jets. 1 is for when the car is tikking over, the other is for drive. Check if these are blocked. They unscrew easy enough, beware that the jets might not be the same size, so do them 1 at a time.
Thats what had happend on my 340.
If it turns out to be an fuel problem, I would sugest you put an inline filter in the fuel hose, only cost a couple of pounds. That way you will be ale to almost rule out the carburettor in the future should the problem return.
Hope this helps