So i have a 1984 f 150 and lately it has been diening at low rpms. I have taken it into the shop have been told its vapor locking. However other people i have talked to said its what they call the brain box. I drove it from fortcollins to denver and it was fine at high rpms but as soon as i hit lower rpms the truck boggs down and dies. sometimes the truck will start back up right away and sometimes it won't.

 

First thing I would do is make sure no vacuum lines have come loose.

 

Vapor lock sounds suspicious, it would be when weather is HOT and we're not there yet, especially in CO. So is this a new problem or been this way since you u got it? If it was the box, it probably wouldn't run at all. These kinds of problems usually come down to something simple or the result of a lack of maintenance. I agree about the vacuum lines. Had a tuneup within a year? Sounds like it could be weak on the electrical side....weak spark = poor start and idle. Or adjust the idle mixture and idle speed to get it right, then see how it runs overall.