Sascha, have you had this issue for a while or has it been recent? Is it affected by temperature?
My '77 with no carb cooling fan and OEM fuel pump has been fine, hot or cold, high and low. Sometimes it'll get a little heat soak going and take slightly longer to start up, but it always does. I do have a fully electronic ignition from a '79, don't know if that makes a difference. Also OEM plug wires.
I'd like to know more about your symptoms. Is it cold starts or hot starts that are causing you problems?