BMW Repair: BMW losing power for a split second., bmw 318i, diagnostic report

Question
Today while driving my BMW 318i (Late '01) the revs dropped for a split second, from about 3200rpm and returned to normal again. It was like the engine just gave up, the radio etc stayed on and the car did not need to be restarted, the only light to appear on the dash was the battery light. This happened twice, 15 mins after leaving home and about 15mins after again. Second time I didn't notice the battery light.
Almost an hour later on the same journey, the revs dropped again, this time enough for the engine to cut out as it was sitting idle in traffic. It took a few turns of the key to get it going again.
An hour or so later, after coming out of the shop I had the same trouble trying to start it up again. Eventually it fired up but only for 30 seconds when it cut out altogether and the warning light appeared ('!' in a circle) and a beep sounded. It started again after 10 mins so I drove it for 2mins and parked it up.

I've asked a mechanic who thinks it may be a sensor but can't be sure. Can you offer any suggestions. Any help is appreciated.

Answer
Vinny, get a diagnostic report from a shop or preferably the dealer (I am more familiar with their readout and fault descriptions). See what fault is stored in the engine computer (DME). Have the charging system checked for proper operation. If the vehicle's alternator is not charging the battery light will illuminate because the car's system voltage has dropped to 11 to 12 volts. (normal is 13-14.5 Volts with the alternator spinning). Let me know what you find.
-Evan