My 1997 Mercury Mountaineer loss power and shut off...the engine would try to start but would not catch.?

The fuel pump was replaced and 3 wks later my truck did the same thing...a 40 amp fuse was replaced and 3 days later my truck would rev high then low while idling and it would eventually cut off....the idle sensor was replaced and the next day my car began reving high, then low and subsequently cutting off again.....my mechanic can not seem to pinpoint the problem....any suggestions?