is not it still under warranty?

is it bogging down w/ ice?
will the enine run w/ out the impeller spinning?

there are other sites ope on the net and abbys that have some real mavens. might try to post/research there

Sounds like there may be some water in the gas tank. The water will form in a bubble in the bottom of the tank, and roll around into the fuel outlet. Put in a bit of dry-gas. Not a lot, just an ounce or two. And, peek in there with a flashlight-ya might be able to see if there is anything else in there.

The fuel caps breather probably filled with frozen water creating a vacuum in the tank. loosening the top will solve that problem

Thanks to all for your responses. Allow me to address each in this post.
solara: It's still under warranty, which is why I've scheduled the dealer to pick it up tomorrow if I can't troubleshoot it before then. I don't see evidence of it bogging down with ice; when I look into the intake chute after it's stopped, it's clear, so I believe it's discharging everything the auger pulls in. Once it has stalled, the engine won't run even without the impeller engaged (as noted in my post, I've had it idling for 25 minutes without impeller engaged without a problem).

rustyj14: I peeked into the tank with a flashlight, but don't see anything obvious in there (other than liquid). I'll try some dry-gas to see if that helps.

akachrisinmass: I always secure the fuel cap tightly, so I'll try to close it a little less snugly this time (but not so loose that fuel spills out)

One new observation I had last night when taking another look for anything else out of order.... I always turn the fuel valve off before rolling the machine back into my garage, and of course I have to open it before starting the machine. When I opened it last night, I heard a kind of suction sound and then the gurgling of liquid for a few seconds, something which I've never heard before when opening that valve. (This occurred BEFORE I opened the tank cap to peer into it.) I might speculate that this is consistent with *something* frozen (either water in the tank or water in the cap breather), which would explain why it resumes operating after sitting in a warm garage for awhile. Hmmm... maybe it works fine when idling outdoors for a half-hour because there's no snow drifting back over the engine, but when I start moving snow, anything that drifts back over the engine may be finding it's way into the breather and freezing there, as has been suggested.

I'll follow the suggestions here and report back my findings. Thanks again to each of you for your responses!

Hmm... I guess my hypothesis isn't right. I just looked at my gas cap (to see if any foreign matter appeared in the breather hole) expecting to see a small hole in the top of the cap, but there is none - it appears to be a solid, unvented cap. This is on a Briggs and Stratton Powerbuilt Snow Model 12000 engine. If there's no vent hole in the cap, what prevents a vacuum from building in the tank that prevents fuel flow... is there some sort of relief valve someplace I don't see? The owners manual says to "tighten the cap" after refueling, so it sounds like they're not relying upon an incomplete cap seal to permit fuel to flow. (Nevertheless, if this approach resolves my problem without creating a fire hazard from spilled fuel, I'll use it.)

Just because you don't see a hole, don't assume the cap isn't vented.
When the problem occurs, loose the cap for a few seconds and then see if it runs for a few more minutes. IF the cap seems to be the problem, replace it.

I guess a possibility is that the cap may have some moisture trapped in it that freezes when in use, clogging the vent? Maybe flush it a bit with rubbing alcohol??