Cause of an occasional rough start?

[Severian]

Our '98 ti occasionally starts a bit on the rough side. It has happened about 7 times since we purchased it a month ago. By "rough start" I mean that immediately upon ignition the car rumbles a bit then konks out. Turn key to the "off" position, count to 3, start her up and give her a bit of gas for good measure, and everything's good. Usually the A/C happened to be on when this happened, so I thought they may be related.

This morning my wife started her car to hit her commute to work. The A/C wasn't on, and the car konked out just after starting. But then successive attempts to start it up failed. The car was turning over, but the sound that it made while turning over didn't sound 100% normal, and it didn't kick into a full start despite holding the key in position for 4-6 seconds at a time. I can't describe the difference, maybe it was all just in my head at the time (because we were running late as it was). But it was turning over.

Finally, my step son came down and while it was attempting to start up gave it some gas. The engine started up, and everything was back to normal. Do you guys have any idea what may cause the occasional rough start, despite A/C setting? (it seems)

We have:
1998 318ti, stock most things, not modded
Manual

My step son said perhaps we got some bad gasoline, because there weren't any other symptoms. We have been feeding it regular gasoline. He suggested we start using 89 octane +

Thanks!!