This sounds exactly like a problem I am having with an '89 FSuper Duty Diesel that is in my shop right now. I will assume that you have a manual trans since you said nothing about a shifting problem. I will make a deal with you....whoever finds it first has to post it here, ok? I have replaced the tach sensor and tried an ECM with no luck. All of the wires check good and the terminals are tight.......I will just keep looking, I guess. | 
