Television and Home Theater

Hi

I have an older big screen that I adore..however in the last couple of years, I lost stations, channels 2,4,5 then 7 and now sometimes 11.  I assumed the tv was slowly dying.  However, I'm now thinking, how could so many other stations 3,8, 10 13, cable stations.. come in so very well, if the tv itself was dying.??

If the tv was dying would I even get these ?  Anybody know?

   Do you think it is the cable?  I have tried to determine if the connection s behind the tv and outside the house  are good and they seem to be.

I have a plasma with 4 1 inch stripes going from top to bottom.what do you all think the problem is?and what can I do to fix the problem?

I am having trouble with a panasonic pt50lcx64 when it powers on it tries to get the picture like going through recycling or discrambling the picture and the picture comes on for a second then it goes to greean and then blue screen then it shutts off please let me know what I can check for troubleshooting.