Have the dealer scan for codes. Sometimes, intermittent problems will store a code in memory. The code says there for a while even though the problem has disappeared. This may give the dealer a clue as to the source of the problem.
Also, get to know Manitoba's new car lemon laws. If a dealer cannot fix a problem after a certain number of visits, you may be entitled to some kind of compensation.
FWIW don't be shy about raising hell about this issue. It is a VERY serious safety concern. Just imagine you are turning left in front of busy traffic and the engine dies for 5 seconds... an accident would be likely.
So, make a big fuss and demand a new, replacement car if yours cannot be fixed. Call your regional GM representative or office and get them involved. Send a thorough description, with any witness reports, and your demand for repair or a new car etc to GM and the dealer by registered mail. Therefore, at least, if things get worse, you have a record that they have been informed.
Also, make it clear that if there is an related accident, you will sue and hold the dealer and GM legally responsible for resulting damage and injury. There is precedent for this. GM has been successfully sued for wrongful death and injury in the past for stalling vehicles.
Also, contact the Automobile Protection Association of Canada. Join them and they could probably help you.
http://www.apa.ca/template.asp?lang=english