Bad reception on 68/78

[dervari]

I am having big problems on the Atlanta/Dekalb Comcast system. On Chs 68 and 78 (analog, obviously) I get really bad reception. Other channels are great. I have a 3.5/3.5 splitter outside the house for my HSI, and a 4 way 7/7/7/7 splitter in the attic. I was told my signal level at the outside is 15db, so with this attenuation I'd still have a goot 4.5db signal at the TVs inside. Since my other channels are good, this leads me to believe I'm getting a good signal.

Any idea why I'm having problems with just these two channels?

For clarification, these channels are normal cable channels - no converter required.