Nick Tak wrote an answer about Jetboil Flash Personal Cooking System on April 5, 2010
The original ignition wasn't as sheltered from weather as on the new flash pcs. The sparking element was a little taller on the original, exposing it to bumps and nudges from moving about in a pack. I believe they shortened and repositioned it to be a bit more effective.
I've never had a problem with my original jetboil at altitude, but I do find myself spending a bit more fuel up there than at lower elevation. In the cold I haven't had issues with my original as long as I keep the canisters warm with a scrap of foam padding with reflective blanket attached.
Hope that helps.