Closed captions go on and off sporadically

It can't be. Because, subtitiles are not generated by the TV. It accompany with transmission.

If the captions are turned off on your TV, they have to be coming from your cable box. The TV is just displaying what it's getting. Check your cable box menu and make sure captions are turned off there. Cable boxes, especially on digital systems, can prevent the TV's caption decoder from operating properly. So they need to be able to provide captioning, and I suspect you've turned it on somehow. Comcast should be able to tell you how to turn off captions if you can't figure out the menus.

closed caption can be blocked by the cable box- make sure cable box is set to allow closed caption.

on the TV, makes sure CC is set to CC1.

be certain you are on a Closed Caption program...all PBS stations are Closed Caption, all the time.

The closed caption is probably turned on with the cable box not the TV contact your provider and they can walk you right thru it

If its a Comcast HD cable box here is how to check and or change the closed caption settings

On the remote press cable then power to turn off the cable box only
then press menu ( red button)

you should get a list in black and white
use the arrow keys to go to closed caption and change it to disabled

after your done press power 2 times to return to tv viewing

If you are using a HD cable box then this would be setup in the cable box itself.

Example:
With a Motorola Cable box, you would turn it off and press the menu button (with the receiver off and TV on). This will bring up a menu that will allow you to adjust your digital closed caption settings.

If you do not have a Motorola cable box that I would refer to the manual or call the cable company.

The TV will not display closed caption with HDMI input. It must either over-the-air (antenna) or composite input for the TV to manage CC.

To get CC with HDMI from your Comcast DVR, you need to configure the cable box. Instructions can be found here:
http://en.wikibooks.org/wiki/How_to_use_a_Motorola_DVR/Setup

If you want CC all of the time, this works well. But since you must power-off the cable box to change the setting, switching CC on and off is a real pain.

Your set-up is similar to mine (Sharp Aquos 46" and Comcast DVR). I have the composite output from the cable box going to a DVD-recorder and on to the TV. Whenever I need CC (usually to catch some mumbled dialog), I just rewind the DVR, and switch the TV input to the composite input. Then I switch back to HDMI to continue watching in HD.

The problem was with the Motorfolla Comcast cable box not my T.V. . I turned off my Comcast cable box, pressed menu on the remote to the cable box. This made a 16:9 ratio appear on the cable box's LED read out, and a bluish screen appear on my television. One of the options on my television was to turn on and off closed captioning. Therfore, I turned off closed captioning and the problem was solved.

I had the same issue... here's what I did.  If power off your cable box, then press the menu button, there's a selection to enable or disable Closed Captioning.  This works for both HD series Motorola boxes Comcast offers, at least in the Minneapolis/St. Paul area.  Hope this helps!
Evan

I have a Sharp Aquos 46" and go a Comcast HD receiver as well, and had the same problem turning off the Closed Captioning. I order to turn it off you need to go into the box user setting by (starting w/ the cable box and tv on) pressing the power button for the cable box then the menu button for the cable box. This will open up the user settings menu. Just toggle CLOSED CAPTION to DISABLED.