sink clog involving vent stack

I've got a 2nd floor (2 story house) bathroom sink that clogs about every 6 months. I can clear it using Liquid Plumber or by snaking the vent stack. I've tried to snake it at the sink trap but it didn't seem to help.

I would prefer to snake it, but that means going on the roof and there's about 6 inches of snow up there right now.

Does anyone have any suggestions as to the best way of clearing this clog and preventing this from happening over and over again?

Thanks for your help,

Matt