Yeast are pretty tough creatures, so even with the temp swings you are experiencing, they will probably do their job but you may not get the ester profile you were looking for. In Chris White's new yeast book he states that yeast can start to express heat shock proteins with a rising temp of just a few degrees over a short period of time... the same is true with decreasing temps, with the exception of instead of making proteins they get sluggish and want to drop out of solution.
I think your beer will be fine, but seriously look into getting a dedicated fermentation space with heating and cooling available so that you can precisely control the temperature of your fermentation. I fell into the same trap that lots of people do and spent money on the wort production side of things and really neglected the fermentation side. Needless to say, I was very disappointed with the results because I had no consistency control of the fermentation.
It sounds like you are doing a water bath, so look into a Johnson or Ranco controller to control your heater so that you can have better control over the fermentation and you won't have to mess with it so much. You can easily find them for less than $100 and I think that if you use one you will be much happier with the results of your beer.