signal changes

About 2 weeks ago I bought a new Sony Grand Wega 42" HDTV- Mediacom came out to hook it up- I only work for 2 days and then received either a (no signal or signal cannot be decoded) from the hd channels- the regular ones are ok- I know the lines in my neighborhood are old and they have had some problems- Sometimes I'll lose the interner because the incoming signal will drift lower. My question is. Is the problem actually with the incoming signal or is it the cablecard or possibly the tv itself. Would auto-programming do anygood or would it actually screw up whatever mediacom has already done. Will I lose the signal altogether to the hd channels? I know this is long, but I'm getting rather upset. Thanks for any input. Pat Sh.