I work on B737's everyday and can't say I have seen the same porthole issues regurally. It can happen in very cold conditions, however if it is as regular as you say perhaps it is something that should be brought to the attention on your airline. If worse came to worse, it would be worth you walking a couple of metres to the rear row and look out the window, better spending 5 seconds looking then popping your slide into a life threatening situation!