How much vertical vent comes off the unit? What is your elevation?

The dealer I've been talking to says its essential to have as much vertical pipe as possible, to create the chimnney effect that sucks air through the firebox for a strong clean burn. When I asked them about a couple of the models they carry that can be set up to vent straight out the back, they said they never actully install them that way, that even a gentle breeze at our elevation would put them out.

Elevation matters because there's less oxygen up here. The flame needs a greater volume of air to get the same amount of oxygen.

If the fireplaCE was installed according to the manufacturer's installation instructions, it should work properly. Did it EVER work reliably?

Just a guess, but I doubt that's it's the winds or the vent at the root of the problem.

Irregular pilot outages are usually caused by a poorly performing pilot light or gas valve that's not working properly.

The things that need to be checked and tested are somewhat subtle. I'd spend time looking for an especially good repairman to check out the equipment and look for weaknesses.