[T-Bird 92]

Hi everybody... I have a '92 5.0 LX w/ four-wheel disc brakes. I've been having an "intermittent" power brake problem- that is, sometimes (randomly) I lose my power assist/ boost. This seems to happen when i'm stop-and-go driving, and not on the highway.
My local repair shop couldn't help (they drove it and couln't reproduce the problem). They told me to take it to the dealer. They said that since the brake booster system was electronically controlled, only the dealership could diagnose the exact problem. I guess this might have to do with using some type of diagnostic/ "code" reader. I was also told that my type of brake system was only used for a couple of years.
I'm sure that someone else must have had a similar problem... any advice?