Secondary fuel system failure 91 Mustang 4 cyl

I have a 91 ford mustang with approx. 93,000 miles on it 4 cyl auto, while driving down the road the car just stops running. No warning, like you turned off the key. When hooked to a computer it comes up with a code secondary fuel system failure according to the haynes manaul which also says beyond the repair of a home mechanic. The car will also shut off sitting in the yard at an idle no specific tme limit sometimes 10 minutes, sometime an hour any help would be appreciated. I have replaced the fuel pump and the air idle solenoid by recomendation of a friend which didnt help thanx NOTE: the car starts right back up. What is the secondary fuel circuit? And what can cause this problem?