Your plaster should not be cracking just due to the cold, you have something else going on that you need to address. Definitely check the attic for leaks.
Also, you had to repair it again because it had pulled away from the lathe, and unless you get someone in there to do a proper fix (including knocking out plaster around the crack) this will keep happening. We have a 110 year old house and many days of 10 degrees or less this year, and I haven't noticed ANY new cracks. An inspector will not catch something like this, please get a properly-trained professional to address your plaster issues. We have one, and he's worth his weight in gold. He has fixed every crack we found after we moved in (in 2004) and not one has reappeared.
What I have to deal with this year is that the guy we hired to replace our gutters and downspouts did a really awful job. I have ice all down the one side of my house from leaking gutters, which he did not properly install so there is a gap between them and the house, allowing the water to roll right off the roof and down the side of the house. I took pictures. I am considering legal action since he won't return our calls.
Our heating bills go up to almost $300/month in cold weather like this (I think our highest bill ever was $245, except for 2009 because my bill was around $400 for 3 months, it must have been really cold that year). We have balanced billing, so I just pay about $100/mo year round.