[-o< i have a 80 f-100 300-6 it hase a new motor and carb some times it runs great some times when i pull up to a stoplight the rpms drop to 200 and comes back up and it cuts out i can mess with the air fuel mix and get it running good agn but when i take a short trip it does it agn it is almost like a vac line is leaking any help will be great