1995 Ford Explorer randomly occuring, brief brake failure

Hi,
I have owned a used Ford SUV (above) since November 2007 and have experienced a strange brake behaviour only 3 times to date.
Pressing the brake to stop at a stop sign and everything feels normal until sudden loss of braking power for a few seconds to a minute without any change in pedal height or any trouble-dash lights coming on.
Pumping the brakes has had no effect and they just become empowered again without seeming to be related to anything I do. At first I thought I was sliding but since there was no response to pressing and releasing the brake pedal at all in my effort to stop the vehicle I realized it's doing it's own thing. I never know when it's going to happen and vacationing in the rockies is out of the question until I correct this fault. Any suggestions that I can tell to my mechanic will be greatly appreciated. Thank-you.
Leslie