Power out on just one section of the house
Thanks for your question, you describe a common issue. When troubleshooting electrical circuits, remember electricity must have a complete path back to its source. This means you not only need a path to the load but a path returning to the source from whence it came. A connection in the neutral path could be loose. 

You are doing well not to ignore this intermittent problem as it points to a probable loose connection somewhere on either side of the path. Loose connections quickly build up heat and can very easily start a fire. 

Since the entire circuit is having trouble, do not forget to check for loose connections at the panel, especially at the neutral bar, and of course the breaker. Try wiggling the wires while you have a multimeter checking voltage. If the voltage suddenly drops you may have just found your loose wire. Trim, restrip, and reconnect. 

Next, pay close attention to the connections at each outlet. Look for obvious signs of heat (melted insulation on conductors, smoke residue in the box), but also for anything that may not be making a good connection. Wiggle the conductors on the screw terminals. If they move around or slip on the screw, voila! Loose connection. Another culprit is insulation that is a little too long and is actually tucked under a screw, preventing a good metal to metal contact between wire and device. Over time, with usage and vibration, the screw may loosen enough to create the intermittent problem you are having. Try tugging on each conductor individually in each wire nut. There could be a loose wire that falls right out of its splice, and bingo! there you are. If you see any of the aforementioned, trim (if possible), restrip and reconnect properly. Sometimes it is better to start fresh with a new tip of conductor that isn't going to break from past stresses. I like about 3/4" of bare conductor laid evenly in wire nut splices or with a nice candy-cane shape for screw terminal connections. Also, be sure to test each spliced connection by tugging on the individual wires to be sure you've got a tight splice. With the screw terminals, think of the candy cane's short end as the pointer and make it point in a clockwise direction going around the screw.

While you are searching for loose connections in the outlet boxes, you can have the circuit turned off for safety; however, another method I've seen involves having the circuit on: Plug in a lamp or something that will indicate when you've got power. Take a piece of wood or something non-conductive and go around poking and prodding into the open boxes and their wiring. If the light suddenly goes on (or off as the case may be) your issue is probably in the box at hand. 

Don't forget light switch boxes, which may have been used as a junction box for your circuit and which get vibration daily. One last place to look for loose connections, a junction box in an attic or basement, although that would probably be the least likely place as it wouldn't get much traffic. 

These are the best recommendations I can think of, although your electrician friend may have already done them all. Hope you find the issue, Nancy, these things can be quite irritating!