Even down to -7C mine started and ran fine on LPG, but it will depend a LOT on the vapouriser fitted as to the problems you get, your symptoms sound like its not warm enough, if it happens again, pop the bonnet and stick yur hand on the vapouriser and see if it is very cold, if so it's definitely the problem.
It could still be your problem however, particularily if it's a sequential system (which I don't think it is, from your description).