I don't think that it is a real problem as mine do it when the temp is a little high and have not had any problems that I can see. It is just their way of getting a little more water/oxygen through their gills. Maybe if the temp stayed high (mid 80's) for more than a week, there may be a problem but I have not had any heat waves lately.
I misunderstood you. I thought the temperature swings were daily!!
Eels are hardy, pretty much the hardiest. Thats not an excuse to not maintain their conditions, but most of them can handle parameters that some fish cant. I would still try to maintain one temperature, or atleast within 3 degrees.