first, if yours is a HD cable box, you would NOT connect it with composite (yellow rca jack) but component video (RGB jacks). component is the only way to get 1080i/720p HD video other than HDMI. composite is std def 480i.
second, try going into your Pioneer's HDMI setup menu and turn HDMI Control off. see if that makes the connection stable. known fact is HDMI Control can be quirky or problematic depending on the specific TV, receiver & how it's implemented. unless you absolutely can't live without marrying turning on/off TV & receiver together at one time, etc, you are better off IMO not having HDMI Control (also called HDMI CEC) on.
third, try a different HDMI cable
fourth, try turning the Video Converter in the receiver off
see if any of these help you.
in the end, cable/sat boxes have been the most quirky with respect to HDMI implementation and a whole lot us don't even bother with trying to make HDMI work with them...I use component video & an optical digital connection and don't look back. HD cable boxes are only capable of 1080i not 1080p anyway, don't carry hi-req audio formats (Dolby Digital only) so no real need to use HDMI other than "convenience". with HDMI, you will also notice a slight time delay every time you change channels.
if your box is a Cisco, I saw with my own eyes a close relative have channels randomly go dark. one time a channel would be available, and 5 min later it wouldn't be. had to call TW & have them remotely reboot the box and let it re-set the channels. she had been living with that for over a month - pretty flaky performance IMO. as much as I dislike Comcast, at least the Motorola HD DVR I have is stable & doesn't "need" rebooting to work. maybe TW would be willing to swap you to a different mfg if all else fails you.